New evidence reveals a “strong possibility” that Black - who died aged 68 earlier this year - was in Paderborn, Germany, when Katrice Lee disappeared without a trace.
The toddler was snatched in November 1981 from where her family were stationed with the British Army.
Last night, Katrice’s father, former soldier Richard Lee, welcomed the breakthrough and said he hoped it would be followed up properly by the relevant authorities.
“It is a new avenue which is well worth looking into,” said the 66-year-old. “We have always said there are two options - that she was stolen to order and sold for profit or that she was sadly murdered.
“We realise there could be a dark side to this story and if that is proven and we have a body, then at least we can have closure and move on.”
Retired detective Chris Clark, 70, who has worked on dozens of child abduction cases, said the two theories put forward previously - that Katrice drowned in a nearby river or that she was snatched by a childless couple - do not make sense.
He added: “The manner in which she disappeared was exactly the way Black had been able to make children vanish.”