"Stormin Mormon" writes:
If a hermetic compressor shorts out (electrical windings)
and burns up, it can decompose the refrigerant to other
chemicals. As to dark oil, not likely. But in any case, the
chemicals are still within the sealed system. They used to
make acid absorbing filter driers, but those are also part
of the sealed system.

Most kinds of winding insulation have some carbon in them, and produce
black soot if they burn. Black carbon in the oil means black oil, so
black oil doesn't seem that unlikely if there's been a burnout.

Dave