1992 Chevy Silverado high oil consumption

I have a 92 chevy pickup with high oil consumption I have already had the heads off of it and took them to the shop to have the valve seals and stuff replaced and the guy that did that said that they were really bad and that was where the oil consumption problem was well its still not fixed and when I start my truck after it not running for a good few hours it blows a little black powder out the exhaust for not even a second almost like dried up burn't oil or something i'd like to get this problem fixed without having to tear down the motor again because I already replaced all the gaskets and everything on the top end?