I thought I heard somewhere (maybe Randy said it during a show) that black balls technically arent completely black, that they are a really deep purple.  That doesnt really make sense to me why a black ball would have purple in it (unless it is a multi-color ball).  But it may explain why you are seeing purple in the track in this situation.

I remember many years ago, a friend of mine had a Nitro R/2 test ball that looked black.  But after throwing a few games, his track was purple whereas the rest of the ball looked black. This ball was used on newer synthetics in a Brunswick center that had just opened.