This is probably the part that should be done first before the second.
The first is to add a STRONG visual indicator for Black Powder. I never really understood why Black Powder pretty much only had a ring and a distortion effect to give away that it's being used. At a distance, a player may have a hard time picking up where this effect has been dropped down in the heat of the moment.
If you recall the Ash Legion, in Ascalon they actually have a smoke field effect that's hard to miss. While this isn't a nerf, this adds a needed tell to help the opposing player make a decision.
The second is this.
Reduce the duration of Black Powder field. Give the thief enough time to Use it once in an open field. But juuuust enough time where it's incredibly difficult for the thief to use it a second time in an open field (Not leaping into the wall).
Because of the reduced duration, REDUCE THE INITIATIVE COST TO COMPENSATE OR SPEED UP THE CAST TIME