We’ve had this happen repeatedly to one vanity-type sink. We take the elbow joint (trap) off and clean out a bunch of black slime, and then it works fine for a few months before gradually slowing down. We can keep using it when it drains slowly, but eventually we steel ourselves for the black slime again.
The trap itself and the pipe going down to it get very clean when we clean them, but there might be something in the pipe that goes into the wall, even though we’ve snaked that with a DIY snake from Home Depot. As far as I can tell, the trap joint and the pipes near it are chrome-plated metal of some kind, but I don’t know what’s inside the wall deeper in.
A plumber we called said “you have the old-style pipes; this will keep happening until you get all new pipes put in,” to paraphrase my wife’s version of what he said.
Does that make ANY sense? It sounds like an attempt to upsell us on a lot of work. Are there such things as “old” pipes that get slimed even if they’re cleaned out, whereas new pipes are immune?