Hard to turn them purple but, here's a link.
Ian

Here is a link that might be useful:

Deepening/intensifying the color or even changing it significantly has as much to do with the specific cultivar as it has with soil chemistry. Some cultivars just never produce very deep or intense coloring and some are rather stable in color and not easily influenced by soil pH.

Do you know what cultivar of bigleaf hydrangea you have? 'Nikko Blue', a very popular and widely planted selection and which yours could very well be, tends to produce only midblue flowers at best under proper soil conditions. Under alternate soil conditions, the most you could hope for is a mauvey-light purple or soft pink flowers. 'Endless Summer Original' will produce similar coloring as do a number of other, less common selections. Exposure will also influence color to a certain degree -- a lot of sun will tend to wash out or bleach flower color; more shade will produce deeper colors. But to get a dark blue flower or a deep purple flower, you need to start with a cultivar that has more darker, more intense coloring to begin with.

The one you have is so pretty the way it is, why don't you just buy another....something with really intense color, Venice, Paris or some other one and then if you want to manipulate the color through soil ph you will end up with what you desire. If you're in zone 8 you can grow anything!! I too, love the dark purple ones!! Good Luck!!

Donna

Gardengal: I think it is Nikko Blue. It gets morning sun and afternoon shade. Last fall, I added a bit of aluminum sulfate but never noticed any change in the color. I was hoping for at least a deeper blue.
Thank you all for your suggestions.