Optimum pH for the typically-preferred blue hydrangeas is between 4.5 and 5.5. Sulf-N^® ammonium sulfate can indeed help lower soil pH.

As for the rate of application, it will depend on the pH and the buffer capacity of the soil. If you submit a soil sample to a soil testing lab, you will be provided with a specific rate recommendation for your soil. Generally speaking, the higher the soil pH, the higher the organic matter level and the heavier the texture, the more Sulf-N ammonium sulfate it will take to lower the soil pH. Depending on the situation, it may make sense to split-apply the total amount of product over several seasons, gradually achieving the perfect blue color.