I'm sure your mind is fine, Ken. That is a real mystery.
Not to doubt the PhD's and all, but could it be a type of moss or algae bloom? Was there snow sitting on that area?
I'd be experimenting (just because I love to experiment anyway) with a watering can and some different concoctions - a baking soda solution comes to mind. Vinegar/water would be another try (works as a weed killer, so not too strong) , and believe it or not, I kill off the dark green moss that grows on my walkway tiles with a double strength solution of MiracleGro!
If you've got a few dry days coming up, have some fun. Probably can't hurt!
