Well if i understand you correctly, you have a wall that is textured black and your trying to make it invisible, but not passable, correct?

All you should have to do is set the black wall to "TRANSPARENT" or "TRANSLUCENT",(what ever it is in A7), or even "INVISIBLE" might work. (not trying to be a smart a$$.

once you do this, remember to rebuild your level. Good Luck.