The black gunk in your bathroom sink drain is most likely a build-up of soap scum, dirt, hair, and other organic matter. Over time, this material can harden and turn black, especially if it's constantly exposed to water and moisture. Fortunately, there are a few simple steps you can take to clean out your drain and keep it looking fresh and new.
Start by removing the stopper from your sink and using a plunger to dislodge any large clumps of debris. If you don't have a plunger handy, you can also try using a wire hanger or similar object to fish out the material. Once you've removed as much of the gunk as possible, flush hot water down the drain to help loosen any remaining residue.
Next, make a paste out of baking soda and vinegar and apply it liberally to the inside of your drain. Let the mixture sit for several minutes before flushing with more hot water. This natural cleaning solution will help break down stubborn grime without harsh chemicals.
If your sink is still looking dirty after following these tips, you may need to call in a professional plumber to give it a thorough cleaning. In most cases, however, a little DIY elbow grease should be enough to get rid of that pesky black gunk for good!