Chris, that thread is way outdated. It's so old the function signature for al_set_blender is different.
PNGs are loaded exactly as they are stored. There is no conversion of anything to anything.
If your bitmap is drawing black, it's probably because your blender is wrong.
The default blender is (ALLEGRO_ADD , ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA) for premultiplied alpha blending.
Try saving the png with allegro. It should come out exactly the same as the png you loaded.