That black growth is usually biofilm, a bacterial slime feeding on soap, hair, and food residue, rather than household mold. Scrubbing the visible part treats the symptom. If it returns within weeks, the buildup runs further down the pipe wall than you can reach, and the drain needs proper cleaning rather than a surface scrub.
Biofilm forms wherever bacteria find moisture, food, and a surface to hold. A drain gives them all three. Soap scum and grease coat the pipe wall, hair gives the film something to anchor to, and every rinse delivers fresh food. The black color comes from the bacteria and the debris they trap, not from mold spores.
Actual mold needs air, so it grows around the drain opening and under the sink, not inside a pipe that stays wet. If the growth sits on the countertop, the stopper, or the cabinet floor, that is mold and it means a slow leak somewhere. If it lines the inside of the drain, it is biofilm.
The film matters because it narrows the pipe. Kitchen drains in DFW homes carry more grease than bathroom drains, and once biofilm holds that grease, the layer thickens until water backs up. Biofilm is still bacteria, so wear gloves to scrub it and wash your hands afterward.
A drain that clears and stays clear after one proper cleaning was a biofilm problem. A drain that backs up again within a month has something else going on, usually a partial clog further out in the branch.
Nuflow DFW is a licensed trenchless sewer contractor serving 25 Dallas-Fort Worth cities, RMP# 46694. Our drain cleaning clears the pipe wall rather than punching through the film, and a camera inspection shows what was in there. You keep the footage. Call (469) 701-0597.