"Fit" is relative. With a grinder and a welder, your options are fairly wide. Lots of guys here putting late-model (90's & 2000's) seats into our trucks.
If you want a drop-in replacement, the front buckets are the same from '69 Blazers up through the early first-gens, like 75-76-77ish. I know my '74 shared the lowback seats with the first-gens.
The brackets, however, may differ, I don't recall. My '74 driver's had forward-backward sliding tracks, and the passenger seat flipped forward on a hinged for rear-seat access.
My memory is that the bolt-down (not flip-up) Blazer rear seat is very close or identical in size and brackets to the similarly equipped Suburban third-row seat, again through about 76-77.
-- A