v3.26.1
Document and Entity Information - shares
 3 Months Ended
Mar. 31, 2026
May 05, 2026
Entity Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2026  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q1  
Entity Registrant Name BlackRock Monticello Debt Real Estate Investment Trust  
Entity Central Index Key 0002049595  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Shell Company false  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Current Reporting Status Yes  
Entity File Number 000-56720  
Entity Incorporation, State or Country Code MD  
Entity Tax Identification Number 33-6595754  
Entity Address, Address Line One 50 Hudson Yards  
Entity Address, City or Town New York  
Entity Address, State or Province NY  
Entity Address, Postal Zip Code 10001  
City Area Code 212  
Local Phone Number 810-5300  
Document Quarterly Report true  
Document Transition Report false  
Entity Interactive Data Current Yes  
Common Class E    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   347,110
Common Class F-I    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   5,600,925
Common Class S    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   0
Common Class T    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   0
Common Class D    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   0
Common Class I    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   0
Class F-S Common Shares    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   2,745,561
Common Class F-D    
Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   0