Bladerunner was being retroactively called part of the cyberpunk genre 20 odd years ago when I was writing long essays on the subject at university.
"Retroactively" is the keyword here. The Internet unfortunately unlocked the door to all sorts of disturbing historical revisionism, even in popular culture. As someone who watched that movie in theaters back in the 80's and was a fan for years, it really frustrates me arguing about what it was or wasn't with people who most likely weren't even born back then. The word cyberpunk, no matter when it was first coined, had never been used in popular culture before the 90's, and nobody ever described these movies as "cyberpunk" back in the 80's, nobody had even heard of that word in those days, possibly other than the readers of some obscure scifi novel no one else cared about. Blade Runner's influence on today's cyberpunk genre is pretty obvious, but it's entirely stylistic and superficial. In terms of the story, it has nothing to with "cyberpunk".
The cyberpunk genre in literature goes back to the late 70s. I think actually the biggest influence on everything in this area is Judge Dredd, who dates back to the late 70s. Megacity One is
the blueprint of the cyberpunk dystopia. All you need to add to Megacity One to make cyberpunk as we know it is computers and Hong Kong at night (specifically mentioned by Scott as an influence on the aesthetic of the movie). I think a lot of highbrow people were probably influenced by
2000 AD, but because it was a cheap British comic it was too embarrassing to admit
But Dredd's writers were smart British yoof who'd read Ballard, Dick and Burgess. The French comic artist Moebius is also a big influence on everything in this area too (he directly influenced Ridley Scott).
You're right that the movie wasn't connected to the genre in the public consciousness till the 90s. But nerds who had read
Neuromancer and
2000 AD made the connection in the 1980s.