I have seen this but obviously only in the snow. The area was where deer were feeding heavily on corn and acorns. There is a plant that has a leaf that stays purple all winter here in Ontario and it usually grows in low lying areas. It also has a red berry at times. I'm not sure if the deer are eating it but they have been known to a lot of different things including poison ivy. In winter, I notice they will dig through the snow to find the ferns that are still green and I notice some other plants that stay green all winter that the deer will flock to. The only thing I can think of is if the deer are feeding on something that is extremely high in vitamin E which is a naturalanti-inflamatory and in high doses will cause one to pee blue.