You can find all sorts of references on Bottle Trees, but most seem to indicate that the practice started around 9th century Africa as a superstitious way to capture the evil spirit roaming around. A bottle was hung in the tree. When the wind blew you could hear the haunting whistles escaping from the the bottle. The spirit was moaning trying to get out. Blue bottles seemed most effective in capturing errant spirits. But I am sure you could produce the same moaning with clear, green or brown glass.
Blue Bottle #1