Here is what Hydrangeas Plus says:

To encourage "bluing" of the flowers, you need to raise the acidity of the soil. Acidity levels need to be around 5.5 - 6.0 on the pH scale. To lower the pH and increase the amount of aluminum in the soil, apply around the hydrangea aluminum sulfate several times at intervals in the spring and again in the fall if the desired color is not achieved. The amount of aluminum sulfate really depends on the concentration. You donÂt want to over do it because aluminum is toxic in large doses.

Hydrangeas Plus

I have blue hydrangeas and I'd love a pink one. I bought a pink one, and even added lime, but it turned blue anyway. SIGH.

Thanks Buford for the info! I have read that aluminum sulfate could be toxic if used in too large of doses. I am kinda scared it will kill my hydrangea! I guess it will be better to try it more diluted. Speaking of you wanting a pink hydrangea, I have a hydrangea that has pink, purple and blue blooms!! LOL

Aluminum is the element that makes the hydrangeas turn blue, but our soils often have plenty of aluminum in them. All that is needed is sulfur to drop the pH so that the needed element becomes more 'available' for plant uptake. You'll need to know the pH of your soil, and determine if it's clayey, sandy, etc. before you can figure out how much sulfur to use.