Hope you don't mind me throwing my two cents in. You are putting down WAY, WAY too much aluminum sulfate. Aluminum can be toxic to plants if the concentration gets too high. It sounds like there are other factors in play that is keeping the plants from absorbing the aluminum. I would check the pH of your soil. If you need to acidify, then put down a light covering of sulfur around each plant in work it in a little bit. This will drop the pH of your soil rather quickly (just a few days to a couple of weeks).

You can have your soiled analyzed at UMass-Amherst for a very small fee. You just ship them a small bag of your soil and you will get the results in just a couple of weeks. You may find that your soil is deficient in some minerals or elements that you were not even thinking about that may be causing this problem.

excellent advice.
mindy

An old trick that my girlfriends grandmother sticks to, she just pushes old, rusty nails into the soil around the hydrangea. And hers are the famous deep-blue.

What I figure is the nails are more of a long-term solution? Like perhaps the additives wash away too quickly, where the nails keep adding for a long time.

I heard two cups per plant of epsom salts should do it.

The trick with rusty iron (nails, old shovel heads, etc) is that it deepens the color a little, but it doesn't change the pink-blue balance the way changing the soil's acidity does. At least, that's what the old timer gardeners on Cape Cod tell me. (Oh wait, all of a sudden I'M an old timer!)

The theory is that some washed-out looking hydrangeas may be happy with the soil's ph but that an iron deficiency keeps them from developing their deepest colors. No idea if it's true, but if you peek under most of my hydrangeas you'll see an assortment of iron junk. No need to push it into the soil, by the way, iron oxide can travel down all by itself and it may rust more quickly if exposed to air.