Or dust from the road. Engines get an oil film on them, from them being an engine and when you combine dust and oil, they look dirty. I tried to find a very minor oil leak in a blown big block Chevy, using florescent dye. When I hit it with a black light after running for a few minutes, the whole outside of the engine glowed.
Did you use any sealer on your gaskets or put them on dry? What head bolts used or studs?
I retorked my trick flow heads, no I didn't back them off. Yeah, probably a pinch above torque specs, but so are most everyone's torque wrenches out of spec.