Odd that this should come up now... my mother (in PA) was just telling me on September 9th that they get this stuff sometimes, I guess she found it again. Hers is under a cherry tree, the same place she finds it other times. I think perhaps there are some kind of bugs boring into the cherry tree (or perhaps woodpeckers going after bugs under the bark) and it's sap weeping. And maybe the excess rain makes the tree produce more sap? Maybe this time of year the sap is starting to flow back down, preparing for winter... Whatever the case, I would say tree sap. It may have built up over the summer and the recent storms knocked it loose.
Aircraft don't dump their waste in the air, and the stuff from toilets would be blue from the disinfectant that they use. They don't have a dump valve where it could be accidentally dumped in flight either. They aren't like busses (and possibly trains) that pull over a dump pit to drain the waste water, it has to be drained into a truck on the ground.
