I have a couple of chips from the MAISON JASSAUDS (now Golden West Casino) in Bakersfield that also turned entirely black on one side. Found 'em in a dirt field behind the casino parking lot, where they were tossed and exposed to direct sunlight for many days or months. Best scientific theory: the inlays are printed on light-sensitive paper. Receipts on thermal copy paper (like from grocery stores and gas stations) will also turn brown in time.

More exciting theory: it's the work of the Devil.