SECURITIES - Summary of amortized cost of securities available for sale and their estimated fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 341,227	$ 350,431
Gross Unrealized Gains	116	53
Gross Unrealized Losses	(32,260)	(36,236)
Estimated Fair Value	309,083	314,248
U.S. Treasury Note
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	46,936	46,937
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2,641)	(3,178)
Estimated Fair Value	44,295	43,759
Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	81,667	81,725
Gross Unrealized Gains	2	4
Gross Unrealized Losses	(5,147)	(5,431)
Estimated Fair Value	76,522	76,298
U.S. Government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,792	16,367
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(881)	(944)
Estimated Fair Value	14,911	15,423
Obligations issued by U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	16,512	16,559
Gross Unrealized Gains	114	49
Gross Unrealized Losses	(247)	(340)
Estimated Fair Value	16,379	16,268
Residential one-to-four family
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	161,428	164,843
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(22,110)	(24,657)
Estimated Fair Value	139,318	140,186
Multifamily
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,368	19,475
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(906)	(1,317)
Estimated Fair Value	13,462	18,158
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,524	4,525
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(328)	(369)
Estimated Fair Value	$ 4,196	$ 4,156

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

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

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
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

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryNotesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=blue_MortgageBackSecuritiesResidentialOneToFourFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=blue_MortgageBackSecuritiesResidentialMultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: