I have a 89 blazer with a 4.3 automatic. Intermittently it wont start and if it will start it is very rough and black smoke out the tail pipe. I let it sit and an hour later it starts with no problem and runs fine. I checked the codes and I get 15 -21 - 44

 

I'm thinking your running way too rich and your fuel isn't completely burning and that is what is causing your black smoke.