That background is black (as in clear_to_color(0, 0, 0)). My code would make my images appear transparent if there was anything behind them. One would imagine that a red block drawn at half opacity over a black background would appear much darker, but in my program, it appears bright red until something is drawn behind it.
Also, given the way I read in color and handle my function call, I do not have direct access to the alpha component (or any individual component). Instead, I just have an ALLEGRO_COLOR object. Not sure how to do what you are suggesting.
I cannot get you screenshots until tonight when I am home again. I am hoping someone can share some insight before then though.