I suspect this is a dual species colony, in which BBA sets up from spores, and is then covered over with GSA or a biofilm.
I agree with your suspicion.
But really knowing exactly what's going on is quite difficult. To make a point, let me show all the different faces of what's typically referred to as BBA:
Easy, classic BBA. But what about this?
Clearly a dual colony including GSA, but since there's no fuzz, is the second species really BBA? Black cyano? Something else? Don't really know.
Still some GSA. But the black stuff is quite different than in the previous picture. Rather than 0.5-1mm spots, it's extremely
tiny; if you were to look very closely at it, you'd find each individual spot is almost as fine as smoke particles! And it shows no preference for leaf edges, either. Is it a different algae altogether? I've also heard people suggest it's deposits due to iron toxicity, or a viral plant infection.
Resembles BBA in color and only. Otherwise, it more closely resembles BGA. Forms sheets on leaf edges, with no visible particles, fuzz, or texture. At the edges where it's thinnest, it's somewhat see-through; and has just a hint of blue-green tint. This I suspect to be black cyano.