The black stuff is actually the very very bottom of BBA. Siamese algae eaters in many cases can't get the very last spot off where it has already grown in many cases. Are you getting it on new leaves? You could also do Excel treatment that might work, but it also might not affect the residue.
I added a new plant since the BBA was under control and it hasn't shown any signs of BBA. So it seems to only be on the older plants that had it. I will have to keep a closer eye on the older plants. Since I added Co2 Injection I have been trying to keep a closer eye on all of the plants. Their growth was kind of bad before; leaves that looked chewed up. The ciclids that I have are leaving the plants alone for the most part so I think it is anemic growth.