Anthocyanins are a common plant pigment. They change from red to blue and purple depending on pH (in fact, anthocyanins were used as pH indicators). The more acidic, the more the color shifts toward purple. I'll pose that the pH in the azalea blooms was made more acidic by the aluminum sulfate (i.e., soil acidity goes farther than just nutrient mineral availability). However, I think the cause-and-effect relationship is that the flowers lost their true color because of an acid deficiency. In other words you can make a purple flower fade to pink (inadequate soil acidity), but you cannot make a pink flower purple (its in the genes).
It might be possible to test my proposition by putting flower stems in solutions of different acidities (analogous to dyeing flowers). I've never tried it, but a pH test kit and dissolving some of your aluminum sulfate in water, might demonstrate this. Another thought is that foliar sprays might work as well. A product called Green Cure is sold for controlling downy mildew. It is potassium bicarbonate, a base (raises pH) that would shift anthocyanin color toward red/pink, if is is adsorbed through the flower stomata. Now, if we just had some blooming azaleas, the fun could begin ;-).
... just an idea.