Hi again, Cynthia,
Your soil must be rich in aluminum for your hydrangea to get this dark blue. If it is not, an addition of aluminum sulfate on the soil, around the plant, will help. I learned also that the choice of fertilzer will affect the color. A fertilizer low in phosphorus and high in potassium is helpful in producing a good blue color
(25/5/30 is good. Potassium is the last number). Superphosphates and bone meal should be avoided when trying to produce blue.
Hope this will help.