Black dust is probably a sign of a sooty environment (using candles?), or any other kind of dust of that color. Grinding dust, say.
The source need not be obvious. After a lifetime of exposure to fresh outdoor air in forest-fire country, I can imagine the accumulated dust being on the dark side. (Someone living in such an area might confirm that suspicion, just speculating.) It needn't be very dusty for very long, nor visibly dusty at all (to any extent that you can see by eye), for visible dust to collect on such things.
Tim