Corey, a hydrangea expert I know suggested I dig the hole for the hydrangeas much wider and deeper than usual, and use a mix of at least half peat moss with the garden soil when I filled it back in, in addition to adding the sulfur and aluminum phosphate. I didn't get them to turn a true blue, or even a dark purple, but I got at least this far, which gives me hope that it is possible:
I've also read that using pine needle mulch helps to acidify the soil, though I question how much effect that would have.
