I believe you are seeking the book "Black and Blue Magic". In the book, it is not his uncle that gives him the bottle with the magic ointment that grows wings, but a guardian angel seeking to reach his own goal in helping his charge.
Though you indicated it had no picture, the cover is listed below along with a brief description.
Black and Blue Magic on Amazon
You'd think that someone with a name like Harry Houdini Marco would be deft and skillful, but Harry could only occasionally catch even an easy fly ball without making some dumb error. On top of that, most of his friends' families were moving to the suburbs. It would have been a long, dreary summer, but then a Mr. Mazeeck showed up and turned out to be more than he seemed. After helping the elderly gentleman, he receives a mysterious bottle with something very special inside as a reward and with careful instructions as to how to use it. The gift of flight.