I forgot which motors are in the 89 tegs, is it the 1.6 dohc? If so then you might want to consider picking up a b18a1 which is from a 90-93 integra for pretty cheap and throwing it in there because you may be able to get one for what a mechanic could charge to get rid of that..... the black around the muffler is normal as its so old and has all the buildup, and it using oil is probably associated with its age. The black smoke however, could be attributed to it running to rich or just because the age of the motor. IMO I would look into a swap before starting to replace things that add up or taking it to a mechanic.... you could take it to a mechanic and see if they will at least tell you what is causing it, on my 93 LS i had the same prob and couldnt get rid of it so I ended up replacing the motor instead of replacing a lot of parts, how much are you looking to spend on another motor/tranny?