v3.26.1
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2026
Aug. 10, 2026
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
Document Transition Report false  
Entity File Number 001-43039  
Entity Registrant Name BLACK SPADE ACQUISITION III CO  
Entity Incorporation, State or Country Code E9  
Entity Tax Identification Number 00-0000000  
Entity Address, Address Line One Suite 2902, 29/F, The Centrium  
Entity Address, Adress Line Two 60 Wyndham Street  
Entity Address, City or Town Central  
Entity Address, Country HK  
Entity Address, Postal Zip Code 00000  
City Area Code 852  
Local Phone Number 3955 1316  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Shell Company true  
Entity Central Index Key 0002087087  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Class A ordinary shares    
Document and Entity Information    
Title of 12(b) Security Class A ordinary shares, $0.0001 par value, included as part of the units  
Trading Symbol BIII  
Security Exchange Name NYSE  
Entity Common Stock, Shares Outstanding   17,250,000
Redeemable warrants, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50, included as part of the units    
Document and Entity Information    
Title of 12(b) Security Redeemable warrants, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50, included as part of the units  
Trading Symbol BIIIW  
Security Exchange Name NYSE  
Units, each consisting of one Class A ordinary share and one-third of one redeemable warrant    
Document and Entity Information    
Title of 12(b) Security Units, each consisting of one Class A ordinary share and one-third of one redeemable warrant  
Trading Symbol BIIIU  
Security Exchange Name NYSE  
Class B ordinary shares    
Document and Entity Information    
Entity Common Stock, Shares Outstanding   5,750,000