The recommended application rate is 1 Tbsp per gallon of water, applied as a soil drench over the root zone. Repeat periodically throughout the growing season, every 6-8 weeks or so. Do NOT increase dosage or overapply. Too much aluminum will create toxicity and can burn the plant. It takes time for the proper chemical reaction with the soil to occur so you might get mixed results this season. And if your soil is is rather alkaline, even this may not be adequate to encourage blue flowers - growing the plant in a container may be advised under those circumstances.

Thanks! That was very helpful information.

i've seen on tv people adding fallen pine needles to the base of hydrageas, is that another option for turning them blue, and how often to you do that? my soil must be neutral, my endless summers are a pinky/blue color

It depends on your soil condition, but in general, mulches - regardless of composition - do not change soil pH to any measurable degree. Mulches have many other benefits but altering soil pH is not one of them :-) You will get some weak leaching of acids from pine needles but typically not enough to affect pH below the soil surface. And as they deccompose, even very acidic mulches like pine needles/straw, oak leaves, etc. become nearly neutral with regards to pH. It is the level of aluminum present in the soil that turns hydrangeas blue. This is just more readily accessible by the plant roots if the soil is already acidic. Aluminum sulfate kills two birds with one stone - the sulfur lowers pH and the aluminum colors the flowers.