Black or blue?
The black stuff is just exhaust residue getting washed out by the condensation.
If you have black smoke when you stomp on it, it's getting way to much fuel and that is why the black is there in the first place.
Smog systems can cause condensation also, since they inject fresh air into the exhaust system and it then causes condensation.
I really suspect the camaro has this issue because of the smog system.