What about a thin, narrow brush to clean in the drain weekly. Or it is easy enough to take that pop up stopper out and clean it periodically.

Thanks for that, but I have tried this, but unfortunately, the nasty stuff ends up spattering out of the drain will doing this, or gets clotted up in the brush (using a bottle brush)...I'm sure this stuff isn't healthy and am trying to minimize contact with it...

You could remove the pop-up stopper and replace with a strainer. Sink strainer

I occasionally remove the pop-up and wipe the drain with a paper towel and some bathroom cleaner.

Use an old toothbrush you can just throw away afterwards. Get some spray bleach. Grab an old towel, spray The bleach, then lay or hold the towel over it while you scrub away with the toothbrush. The towel well keep it from splattering out on things. Truly the only way you’re going to get rid of the black stuff.

This is very strange- I have this issue in one particular sink in the house. It's in the master bath which has 2 sinks and only one has this. I have never seen this before (we moved in last summer). At first I thought it was something from the previous owner, but it's the strangest thing. It is really gross.

What does it come from? We are on city water no septic.

It's a bit of a chore to do since you have to turn off the main water source; however, if the gunk keeps coming back, I'd suspect something is growing in the P-trap, so I'd remove and thoroughly clean that. If you can't get it clean, I'd consider replacing it. (For those unfamiliar...The P-trap is the little U-shaped pipe that prevents sewer gasses from wafting out of the drain.)

salonva: this is very, very random, so ignore if doesn't apply to you. We had the same thing...few years ago. My sink, master bath, no other sink in the house. I racked my brain about why it was slow draining, and had black sludge, when I don't do anything but brush teeth and wash hands there. No hair down sink. What could be causing it?

I finally figured it out. It was diaper cream, that I'd have to wash off my hands when the kids were babies. Maybe it's something you're washing off your hands.

(I know, it's kind of gross).

Larri...I have a terrible time with mold in this bathroom but I will try checking the trap...The second bath is rarely used, so that may be why I don't have this problem in there...

Mrs. S...There is nothing like that going on in my home...no diapers, children, etc...just dogs, and they don't use the sink...at least not that I am aware of...LOL...

You most likely have a build up of sludge inside the drain pipes.

I would disconnect the drain pieces and take them to another sink or outside and clean them. Bleach or quaternary disinfectant, bottle brush, etc. And clean the fixed open drain pipes below where you removed the undersink drain pieces best that you can.

Re the 'terrible time with mold in this bathroom'--do you have a bathroom exhaust vent? Do you use it religiously? I have mine set up on a 30 minute timer which I reset to run the full 30 minutes after I'm finished showering and leave the bathroom. (You can also buy exhaust fans with a built in humidistat.)

In the future, it wouldn't hurt to pour a capful of chlorine bleach in the drain every couple of weeks to kill the nasties breeding in the pipes.