Black, who was convicted of murdering four young girls across the UK, died in Maghaberry prison last month.
The detectives were in Northern Ireland last week to question Barry McCarney who is serving life for the murder of 15 month old Millie Martin in 2009.
They questioned McCarney twice over three days, for four hours each time.
After Black's death in January, McCarney told prison authorities that the paedophile had confessed several murders to him.
Although Black was convicted of four child killings, he never admitted any of them.
Timeline of Robert Black's killings
The only crime which Black admitted to was the abduction of a girl in his native Scotland when he was caught red-handed.
The BBC understands Black and McCarney became close while they were both housed in Maghaberry prison's hospital wing.
Black is suspected of at least 12 other unsolved child murders, including that of Genette Tate in Devon in 1978.
She disappeared while riding her bicycle on her newspaper round. Her body has never been found.
Detectives from Devon and Cornwall Police were close to charging Black with her murder when he died.
The force, however, has not confirmed if the detectives who visited McCarney came from there.
Meanwhile, in a separate development, a former RUC detective who said he found the crucial piece of evidence which helped convict Black of Jennifer Cardy's murder has been speaking to the BBC.
He said he found a petrol receipt which proved Black was less than a mile from Jennifer's house on the day she disappeared.
That was 16 years before Black stood trial for Jennifer's murder and a delay which the former officer said was unacceptable.
"We went to the headquarters at Shell at Worthenshaw and it was there we sat in the room with monitors and we were looking for Robert Black's signature coming up where he had got petrol.
"Whenever we found the receipt that he was only less than a mile from McKee's Dam on the Hillsborough Road where he got petrol.
"We were able to tie him down that he was in Northern Ireland at the time Jennifer Cardy went missing."
Claims that serial child killer Robert Black confessed several more killings to a prisoner in Northern Ireland are being examined by English police.