My question is what causes black dust particles? In my master bathroom, this is a constant problem, no sooner than I clean up the particles, they are back again, I have cleaned out our bathroom vent, it has collected dust as well, this does not occur in any other bathroom in our home. There is an opening to the attic in a closet ceiling right by the bathroom entrance. Can someone give me some idea as to what causes this.

Having not seen it this is a guess.
I have done rehabs and it seems as though plaster homes have this sort of black dust accuulate on it/ behind it. It makes having a respirator a neccesity when taking it down. This is not uncommon, it is always. It may just be a collection of dirt in the air from decades of existance. Possibly getting in to your attic from vented eaves. Maybe from a highway if you are near one. But it is always there. I guess as your furniture picks up dust so does your attic only that have never been wiped or swept either, like any other floor in your house.

As a 12 year home inspector I have seen many 'things' in homes and bathrooms. Black dust could be several things. One, it is possibly a mold-like substance since mold-like substances such as mildew is a common occurance in bathrooms, even if it has a fan (make sure the fan is venting to the EXTERIOR of the home and not just to the attic or space above the ceiling.) If it is a mold-like substance (and that ,can only be determined by having a mold test conducted), then proper venting is curcial to remove the excess moisture. You mentioned that there is an opening to the attic near the bathroom. A home is a dynamic structure. One issue many people do not consider is pressurization. When a large fan is activated (such as a whole-house fan or the air handler-forced air furnace/a/c) the home becomes pressurized and does either one of two things-it either blows or draws air. Menaing, it either blows air out of the home if the home is not very tight or it draws air in if the home is somewhat tight. Either is applicable in this case. When large fans in the home run they may be drawing air from the attic, basement, crawlspace, ect and discharge it into one or more rooms. The 'black dust' could be coming from the attic through the access and then pulled into a moist area (such as the bath) and stick to the walls where it may just be dust or could be a mold-like substance that then grows. This issue could be resolved by having a professional evaluate the home to determine just what is happening. You see, there is rairly one simple answer to even a seemingly simple issue. Just remember, a home is a complicated dymanic structure that breathes, expells air, ***** in air, holds in air, draws air in from any source of least resistance. Good luck