Black sludge can easily build up in the drain of the bathroom sink from mould and mildew development, as well as toothpaste and other particles you rinse down the sink and thereby allow to accumulate in the drain pipe. A moderately easy process, cleaning away the black sludge will take care of the blockage and leave your sink draining properly.
Bring 1/2 gallon of water to a boil, and pour it down the clogged drain. The extreme temperature of the hot water will often break apart the sludge and remove the blockage.
Plunge the sink five to six times with either a toilet plunger or a small handheld plunger. This will loosen up any remaining sludge, making it easier to rinse down the drain.
Pour 1 cup of baking soda into the drain. The baking soda will deodorise and clean the drains while working as a nonabrasive agent that will break up the black sludge. Pour into the drain and let sit for a few minutes.
Pour 1 cup of distilled white vinegar down the drain onto the baking soda. Cover the drain so the ingredients can work their way down through the pipe. The white vinegar and baking soda will create a foaming reaction, which will work its way down the pipe to disinfect and break apart the sludge, rinsing it down the drain with little work required. Both the vinegar and baking soda work as deodorisers, which will also eliminate the foul odour in the drain caused by the sludge.
If problems persist, purchase a pipe snake from your local hardware or home improvement store. Run the snake down the pipe, which will break up and wash down any build-up in the neck of the drain, where blockage often occurs. This will leave the drain clear of blockage, and draining properly.
If none of the above solutions work, consult with a plumber, who will be able to handle the problem professionally.