With the loose end cap moving around against the other
aluminum housing, it is polishing the two parts where they
contact each other. If you have ever polished aluminum you
know what happens, the polishing cloth gets black really
quick. So is the oil, as it flows over this area that is being
polished by movement of the aluminum parts polishing each
other.

I'm not good at explaining things, but believe me, this condition
will definitely turn the spent oil black.