When he saw a little girl being bundled into a van, a sharp-eyed retired sub-postmaster took down its registration number.
It was a sunny July day in 1990, and the chance sighting by pensioner David Herkes in Stow, in the Scottish Borders, sparked a massive search.
After stopping to sexually abuse the terrified six-year-old in a lay-by, Black returned through the same village. Police were waiting and pulled him over.
When they opened the doors of the van, the child was lying trussed up in a sleeping bag, with tape over her mouth, and barely alive.
Black’s appalling catalogue of crime leaves little room for doubt as to what would have happened had it not been for that stroke of luck.
Cruising the length of the British Isles and beyond in anonymous-looking delivery vans loaded with posters and billboards, the paedophile was on the look-out for lone young girls to abduct.
Targeting children out on errands or riding their bikes, and always during the school holidays, he almost invariably sought out girls wearing skirts and white socks.
Black was jailed for life for the 1990 kidnap, but it was the crucial discovery of petrol receipts from the firm where he worked that enabled a dogged team of detectives finally to convict him of murdering three schoolgirls.
Susan Maxwell, 11, was abducted as she walked home to her family’s farmhouse in a remote part of Northumberland in July 1982.
She had been playing tennis with a friend, and had persuaded her mother to let her walk home by herself for the first time.
Two weeks later her body was dumped 264 miles away in Staffordshire. It was the beginning of a grim pattern.
Twelve months later, five-year-old Caroline Hogg vanished after going to a playground near her home in Edinburgh.
In a princess dress she had worn to a friend’s party, she was allowed out for only five minutes. Her naked body was found ten days later in Leicestershire.
In March 1986 ten-year-old Sarah Harper was abducted on her way home from a shopping errand for her mother in Morley, near Leeds.
Her body was found more than three weeks later in the River Trent near Nottingham, within what detectives were now describing as the ‘Midlands triangle’.
Following Black’s capture, they discovered he had a long and troubling history of abusing young girls and hoarding child pornography.
Black’s twisted interest in young girls began when he was just 12 and was accused of trying to rape a young girl.
He was sent to a children’s home and then to the boys-only Red House care home at Musselburgh, near Edinburgh, where he was abused by a staff member.
In 1963, months after leaving care at the age of 16, he molested a seven-year-old girl in an abandoned air raid shelter on the pretext of showing her a box of kittens, choking her to within an inch of her life.
It was the start of a campaign of attacks during which he admitted to ‘touching up’ more than 40 young girls over three years, eventually being sent to Borstal in 1967.
Then came an opportunity to start afresh – he met a respectable Scottish couple, Edward and Kathy Rayson, who took pity on his tale of a loveless, deprived childhood.
They invited him to move into their home in Stamford Hill, North London, with their five children
, and he lived there for more than a decade.
It was only after his arrest that their children, who by then had left home, revealed that Black had alarmed them by trying to show them child pornography and hoarding girls’ swimming costumes in his bedroom.
His warped sexuality was also made plain when he was sacked from his job as a lifeguard at Hornsey baths for fondling young girls.
It was his landing of a job driving a van for the Poster Despatch and Storage Company in 1976 which gave him the opportunity to turn his sinister fantasies into grim reality.
The lonely job meant regularly criss-crossing the British Isles and even travelling into France and the Netherlands.
Black would take with him a mattress plus a horrific ‘kit’ with which to truss up any girl who had the misfortune to catch his eye.
It later emerged that he had apparently dumped the bodies of Susan, Caroline and Sarah on trips to visit the trusting Raysons, who by then had moved to Leicestershire.
Last night survivor Teresa Thornhill said her ordeal still haunted her 23 years on.
Now a mother of two, she said: ‘I see his face almost every day. I can smell the oily hands. I can picture the clothes he was wearing. It just never goes away.
‘I sometimes feel so guilty because I’m here and they’re not.’