LOANS RECEIVABLE, NET - Summary of past due loans in non-accrual and past 90 days still on accrual (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Past Due [Line Items]
Non-accrual	$ 6,118
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	6,118
Nonaccrual		$ 7,767
Loans Past Due 90 Days and Still Accruing		61
Residential Portfolio Segment \| Residential one-to-four family
Financing Receivable, Past Due [Line Items]
Non-accrual	5,884
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	5,884
Nonaccrual		7,498
Loans Past Due 90 Days and Still Accruing		0
Residential Portfolio Segment \| Multifamily
Financing Receivable, Past Due [Line Items]
Non-accrual	146
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	146
Nonaccrual		182
Loans Past Due 90 Days and Still Accruing		0
Non-residential \| Non-residential
Financing Receivable, Past Due [Line Items]
Non-accrual	0
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	0
Nonaccrual		0
Loans Past Due 90 Days and Still Accruing		0
Construction \| Construction
Financing Receivable, Past Due [Line Items]
Non-accrual	0
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	0
Nonaccrual		0
Loans Past Due 90 Days and Still Accruing		0
Junior liens \| Junior liens
Financing Receivable, Past Due [Line Items]
Non-accrual	49
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	49
Nonaccrual		52
Loans Past Due 90 Days and Still Accruing		0
Construction \| Commercial and industrial
Financing Receivable, Past Due [Line Items]
Non-accrual	39
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	39
Construction \| Commercial and Industrial
Financing Receivable, Past Due [Line Items]
Nonaccrual		35
Loans Past Due 90 Days and Still Accruing		$ 61
Consumer and other \| Consumer and other
Financing Receivable, Past Due [Line Items]
Non-accrual	0
Interest Income Recognized on Non-accrual Loans	0
Amortized Cost Basis of Loans 90 Days Past Due and Still Accruing	0
Amortized Cost Basis of Non-accrual Loans Without Related Allowance	$ 0

Amortized cost excluding accrued interest, of financing receivable 90 days or more past due and still accruing. Excludes net investment in lease.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-16

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-16

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of interest income on financing receivable on nonaccrual status.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-16

+ Details

Name:	us-gaap_FinancingReceivableNonaccrualInterestIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amortized cost of financing receivable 90 days or more past due and still accruing. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 80
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-80

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-16

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (b)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-7

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-16

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (3)
-Publisher SEC

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (2)
-Publisher SEC

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-7

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-14

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 80
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-80

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=blue_OneToFourFamilyResidentialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=blue_NonresidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=blue_NonresidentialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=blue_ConstructionPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=blue_JuniorLienPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_JuniorLienMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=blue_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=blue_CommercialAndIndustrialIncludingPPPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=blue_ConsumerAndOtherPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=blue_ConsumerAndOtherLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: