GoF: why didn't Sirius know about Snape?

[Rita Winston]

No: HPFGUIDX 6266

(IIRC) While Sirius was in Azkaban, he heard plenty from the convicted 
Death Eaters crying out in their nightmares. He heard enough to say 
that Peter was hiding from his former colleagues, who harbored 
unkindly feelings toward him for having led their Dark Lord to 
destruction. He heard enough to say that they were not happy that 
Karkaroff had gained his release by incriminating others. But he said 
that he had no idea whether Snape had been a Death Eater, only 
that he used to hang out with people who became Death Eaters. 
Karkaroff knew that Snape had been a Death Eater so surely other 
prisoners would have known it, too. Dumbledore said in open court 
that Snape had turned against the Dark Side and been a spy for 
Our Side, how come none of the convicts knew that and mentioned 
it along with Peter and Karkaroff?