1996 Chevy Lumina Ohh, problems.

My check engine light came on yesterday on my way home from church. It eventually stalled. Thinking I was out of gas, I put some in, then it slowly began to run, but very jerky and hesitate-ish. For example, after I got it home, I revved the engine, and it revs, then falls back, revs, then falls back, never reaching a high rev. After looking around, I noticed black, oily sludge-y type liquid in my radiator overflow (reservoir). Does this mean my engine is bown, or perhaps a cracked head? Does the coolant system need flushed? Please help, as I start back to school, and my internship this week.