if there is any MDxx present it will turn the sample "black" and mask any other reaction.
If this happens, and you suspect the pill to be a k pill, with no apparent MDxx effects, try breaking it in half and testing from the inside, as sometimes MDxx powder can get on the outside and contaminate the test. This can either be accidental, from simply being stored in the same baggie as a MDxx pill, or a deliberate attempt to foil the tester.