Reaction/mechanism for BCIP+NBT in immunoblotting

[root]

In article <3kt7fb$1b22 at bubba.ucc.okstate.edu>, 
root at wentz1.reslife.okstate.edu says...
>
>        In the reaction between 5-bromo, 3-chloro, 4-indoyl phosphate 
>suspension (BCIP) and nitroblue tetrazolium(NBT) which is catalyzed by the 
>enzyme alkaline phosphatase, what is the reaction/mechanism for this 
>reaction?  Does anybody know where I can find any journal references 
relating 
>to this reaction?
>
>                                Sincerely,
>                                
>                                     Undergraduate desiring xtra credit...
>
				      Paul Ferry, OSU