Yuck! Is this black oily scum definitely coming from the water? If your drainage is a bit slow due to clogging, it could be that this black stuff is coming from the clog themselves... and yes, you could possibly have both clogged at the same time if they haven't been cleared for many years! When these "drain rats", as I call them, are removed it's usually shocking just how black and oily they are... otherwise, I'm not sure what the deal would be with the water would be? Sorry if I'm not too much help, I just wanted to get you started - there's going to be somewhere in the region of 2000 years of experience on this forum to help you out, though, so hang in there!