it sounds like algie to me - I've not had my 79 long enough to run much through it - but when i got it the tank was full of algie. The filters had what looked like black sand in them and the bottom of the tank had 3/4 inch of tar like sludge that needed to be cleaned.

I did what I could but I suspect I still have more in there.

In any case - black gunk in the filter sounds like algie to me.