Angie, I feel your pain about the hydrangeas. I have inherited a few mature 'Endless Summer' hydrangeas at my new place and I can't stand the light cotton candy pink blooms. I like my pinks dark, more in the fuchsia-magenta range. I have treated the soil around them with both garden sulfur and aluminum phosphate, but I will have to wait until next year to see the results, if any.
The problem with a lot of soil in this region is that not only is it alkaline, but it has tremendous buffering capacity. By the time you apply enough sulfur or aluminum phosphate to really acidify the soil, you will probably have fried the plants.
