Problem Solving - 1992 S-10 Blazer

I have a 1992 S-10 Blazer - 2 wheel drive. It has recently started running a little ruff. It is harder to start. Use to, all I had to do was turn the key and it started right up, now I have to hold the gas down for a second with the key turned. It runs a little irratic at idle. The check engine light comes on after running for a little bit. It seems to be running a little rich, because it smokes when I first crank it up. It didnt' use to do this. If you let it idle and look at the tail pipe, it has a black mixture of oil and gas maybe? When I take off at intersections, it seems like it loads up and then it will clear out and run fine. Any suggestions?

Look at your serial number and see if the 8th digit is a W..or open the hood and look at the top of the engine, does it have a big intake manifold on it? (Reason I asked is those trucks came with 2 different engines)
If it is a W engine, what has happened is the CPI injector or the fuel lines feeding it have begun to leak inside the intake...this puts raw fuel in the intake causing your rough running, black smoke and SES light to come on.
I would recommend replacing the CPI unit and the lines (GM calls the lines a nut kit).

What are the chances that it is the Oxygen sensor or EGR Valve?

What you are describing is a classic case of the CPI unit leaking.
If you pulled the codes, you most likely will get a code 45 rich exhaust.
Could an EGR cause what you are describing...possibly but with the black smoke on startup and when accelerating, I would lean towards the CPI unit.
Remove the duct that goes to the intake from the filter, turn the key on (don't start the engine) and open the throttle blade...if you smell raw gas than the CPI is your problem.

Thank you GMMERLIN. I will try this. Is the CPI unit and the lines expensive to replace and is hard to replace? Thanks.

They are not cheap...the total cost for both parts are in the 3-400 dollar range.
You will need the CPI unit, nut kit and a plenum gasket.
The most difficult part of the repair is removing the retainer screw for the nut kit.