Somewhere near the soil pH reading of 7, blooms turn a light blue to green to white color that others have described before as light blue or a faint blue. It seems you need to acidify the soil more or more often. Get a soil ph kit to help guide you. Right now, you are doing this a little blind as you do not know how acidic/alkaline the soil is. You want to go further down to readings below a 6 and apparently you are above that. As always, make sure you do not go overboard with amendments as they can burn the roots if you apply too much too fast. Liquids amendments will work faster but have to be reapplied more often. Solids are slower acting; I cannot tell if yours just hasn't kicked in but say, in 1-2 weeks you would have noticed a soil pH change. Make sure you are not overwatering either as this sometimes can make the shrubs be unable to absorb minerals. I get heavy rains in the Fall and this triggers some hydrangea leaves to turn light green for a while (but then they self correct without amendments).