Is the whole pbulb that color, or just one side? Just curious. Is the pbulb firm? I do have a Wilsonara that has one pbulb that on one side is brownish black, the other side is green - too much light too suddenly (the side of pbulb facing the light turned that color). That happened a few years ago, and it's not changed, so if that's what happened to yours, I think it will be fine.
If pbulb is very soft or mushy, then it's likely rot.