Looks fine to me, however you should try to play a bit with that alpha value and see how it goes. Put it 0.5, then 0.0 and so on.
That is actually what I did all alone in my despair, but it gave me no clue at all. Maybe this is hardware dependent after all, I'm currently working with a laptop borrowed from my gf which uses a graphic chipset instead of a true video card, it may simply not handle properly this type of blending operations. Tough luck !