Black soot from water heater
When it gets bad, it just gets worse.
Good info in the threads on this.
It's a sign of not enough air/oxygen. Even if your supply tube doesn't look
like it's obstructed, clean it anyway. Once it starts producing soot, it accumulates in the supply tube and builds up both in the tube and in the
air slots on the adjustment.
(there are also a couple of screws that protude into the tube and they also catch soot and other stuff)
Mine was burning yellow(not enough air). I cleaned it good and now have a nice clean blue flame. The air is easy to adjust by sliding the slotted sleeve. Too much air it roars and burns erratically, too little air and flame goes yellow.
Good luck. The advice here worked for me.
Life is short. Play hard.