John Michell, writing in Philosophical Transactions, in 1784 (!!!) hypothesized that if a star was big enough its gravity would preclude the escape of light: basically a black hole.
His logic isn’t quite the same as we use today: he calculated how escape velocity would increase as a star got bigger.
Via Ptak Science Books.
BTW: This is a blog I’m always ready to drop from my RSS feeds, and then it shows up with pure gold.