As Geistware said, black gunk coming from the engine is a bad sign, a bit of a brownish color is normal. The black stuff is usually metal particles, and it means something is eating something else. The most common source of this is the muffler tail cone starting to losen up. I've also seen it from some of the bolts or screws on the carb.

An an ABC engine, the very first sympton of overheating in the air is power loss. The engine just slows down a tad from where it was when you took off. So if you take off with plenty of power, and notice the power reducing when flying, you are too lean, and you should land and richen the mixture.