You are looking right up the exhaust pipe. What comes out the exhaust? Well combustion byproducts, of course. Also the lubricating oil that you mixed with the gas and has gone through the engine and done it's job. Also wasted gas from incomplete combustion. Also cooling water that has done it's job and now is emulsified with all the exhaust crud. All that mixed together makes a black nasty looking mess. It is normal unless it is excessive. "Excessive" usually means an improper running motor. Or a normal motor running in a bucket.