Reference for Niddah 39:17

כי פכחותא דדיותא קמ"ל כי חרותא דדיותא

R. Eleazar ruled: [A discharge that has a colour] like that of a black olive, pitch or a raven is clean; and it is this that was alluded to in what we have learnt: IF LIGHTER IT IS CLEAN.

Jastrow