It still looks like the black stuff is from external source. The gasket for the camera cover could not have left that much mess. This is quite strange and I would still have bet it was like eye makeup or a leaked pen but the OP insists that it was not there when shipped but was returned that way. There is nothing inside the iPhone that is that black, even the battery guts do not look that black.
A mystery that I hope the OP will come back and help solve!