I live in a suburb of Buffalo, NY and we always see flocks of birds feeding on the lawns. Yesterday, I answered the front door to let my friend come inside and she got pooped on her shoulder. It was a black/blue dropping that also sprinkled onto my front screen door and on several neighbor's cars. Never seen anything like it.
My geese exhibit this every now and again, and it can be quite curious. The only thing I can think of that would cause this (in our case) is feeding them black grapes, which I did tonight. Here is the result after napping on the stoop for a couple hours!!!