Haven't heard of p-heart turning black. I know it will darken to a deep purple in natural sunlight. I got me tempted to put a small block of it outside for a while just to see what it does over time.
It takes a good bit of hand sanding to get that outside aged layer off of purple heart. It may have reacted with the steel blade being in the weather and all. Wish I could help you out more, sounds interesting though.