Drivers side running rich maybe O2 sensor problem?? Please Help!

I've got 94 RMW with the LT1 with 212, X X X miles on it. It runs great and has been maintained very well.  The other day it started running a bit rough and a saw there was black smoke coming out the drivers side exhaust. The SEL light comes on and off every now and then but it still acting weird. There was black smoke out the drivers side exhaust so I figured it was rich on those cylinders.  I haven't gotten a chance to check the code yet cause it always turns off after a couple minutes.  I checked all the spark plugs and all 4 on the drivers side were black but the passenger side all looked good. I checked the spark of all of them and the wires and they all worked. The PO had replace the plug wires too. I checked the compression of all the cylinders too since I had all the plugs out. They were all around the 200 range. I also got some fuel injector cleaner additive for my gas and put some in. I cleaned all the plugs off (not gonna replace them if they're just going to get fouled again but I will when I fix the problem) and tried some of the injector cleaner and it's running a little better but the SEL still comes on and it's still rich with black exhaust and the drivers side. It's still rough though nothing like normal.

So, good spark and good compression means it's probably something with fuel system or emissions.  I'm hoping it's the O2 sensor that's bad. It's be and easy inexpensive fix.
Anybody have any thoughts or ideas??

Thanks, Joe

If it is the O2 sensor it should run rich on all cylinders (IMO). Try reading the trouble code first, that will put you in the right direction.
Pete

You do not have to read the code when the light is on, it is stored for about 50 starts. Since there are separate O2 sensors on each side, it would be logical to assume that the O2 sensor on the given side controls the mixture on that bank of cylinders. You would not want to have one bank to control the opposite side that it is not measuring.