I put plastic behind a shower wall will it trap water and make black mold

More likely will prevent mold. You place a vapor/moisture barrier up before the wonderboard. Wonderboard, cement board, Hardiebacker, etc... will not deteriorate from moisture but they can allow moisture to go through them so you need that barrier. I recently referred to the as water proof but Bljack reminded me that "proof" is not the true or correct term.