Hey Mark! I do see that from time to time. Are you putting in bicarb, soda ash or cal hypo? It may be some undissolved stuff sitting on the floor. If the pH is higher it can cause issues with dissolving chemicals, and colder water does the same. If it's black it's more than likely stuff coming through the screen. I've seen that a bunch. It's always a great add on service to offer birdcage power washing because it makes pool maintenance easier for us as well. Do you notice there aren't any gutters on the house? If you have a roof dumping into the pool that certainly could be it as well.
Have you seen this with screened in pools? You vacuum, you clean the filter, etc... but every week there is a fine powder (sometimes black) at the bottom of screened in pools? Does the screen need to be pressure washed? Is this runoff from the roof? I'm just wondering if you have seen this and what your thoughts are.