Blue Hydrangea Blooms in the making. I always get a kick out of seeing light green blooms slowly turn blue. It’s all about the PH of the soil. A soil PH of 5.5 will produce the blue pigment. Neutral of alkaline (Aluminum) (6.5 or higher) will produce pink flowers. PH values of 5.5 to 6.5 will produce purple flowers. Of course its much easier to produce these PH values in containers than in regular soil. This is why many who buy pink hydrangeas plant them in their yards and have them wind up blue. Thanks for the look and have a great evening.