This is a fairly common mistake done by inexperienced rv'ers....especially those absolute first timers. Rv black tanks require water to move the solids from the tank to the sewer....no water, no movement (no pun intended). Leaving the black tank discharge valve open allows the small amount of liquids to run to the sewer without moving the solids. Therefore, the solid build up in a pile until it blocks the tank.
The requirement is to keep the black tank valve closed until 2/3rds to 3/4ers full, then open the valve to empty. Adding additional water to the tank to make it full, will help.
Ron