Sodium permanganate is the active ingredient in Bluapple® and it can leave a very difficult purple stain. This is, of course, one reason that we advertise not to open the packets. Here are some things that can work.
1. Try a mild acidic solution such as vinegar or lemon juice.
2. Use sodium metabisulfite to remove sodium permanganate stains from all surfaces. This is a preservative found in most grocery stores in the canning supply section.