Black water tank completely full and won't drain. Help pls.

When you get it sorted out you need to go to a marine store & buy a sign to put behind the toilet that says "Don't put anything in the toilet unless it has been eaten first".

Also, never let the tank get full again. There comes a point where you have no wiggle room to use a pressure wand or do anything without overflowing. Not a good situation to be in.