Fidelius Charm

[johnbowman19]

No: HPFGUIDX 90796

I have been re-reading PoA and a new thought occurred to me. DD 
performed the Fidelius Charm right? Why wouldn't he testify to 
clear Black's name? DD presumably would know who the secret keeper 
is, because it is necessary to the Charm to have a secret keeper. 
If DD can clear Snape's name by testifying why can he not clear 
Sirius' name? DD should have testified as soon as Sirius was caught. 
Why did he not? There may have been the problem of all the people 
Peter killed, but Sirius could have explained about MWPP's 
transformations. Surely a life sentence in Azkaban would have been 
more than enough reason to expose his illegal transformations and 
his friends. Sorry if this has been discussed before, if so then 
someone please tell me how to find the previous conversation. 

Thanks
John