You know, it's really weird cause I was thinking about it recently too. 'Cause gray seats are everywhere in the salvage yard and black is rare. I can't see why the covers couldn't be removed, washed, dyed, then washed again to get the desired effect. Just remember, you're probably not starting with a white seat so the final color will be tinted by whatever the current color is.
Of course that assumes you can get the factory covers off. I tried to remove them from my original seats to weld the seat frame back together (or replace it) and had no luck. Got all the hooks, straps, and zippers undone and could only lift the back up about 3 inches. That took so much pressure that the seems were popping.