There is nothing wrong with your Hydrangea. It is normal for most white blooming varieties(cultivars) to have pink or blue tinted flowers near the end of their blooming cycle.
The hue, blue or pink, is determined by the pH of the soil, in which it is planted. Any newly emerging flowers will be white. Next years blooms should emerge white, right on schedule!
If you would prefer that the blooms fade to pink hues, instead of blue, try adding a cup of Dolomitic Lime(powdered type) to the soil around the plant in the fall and again, in early spring. This will increase the pH of the soil. May have to be repeated, if you you have very acidic soil.
No worries, enjoy Mother Nature at work!
Rb

This is a picture of my white Mme. Emile Moullier as it starts to age and get it's blue eye. That is followed by the very light blue tint. It is like having 2 plants in one! Enjoy.
Mary/Gainesville, GA

Thank you razorback33 and goswimmin for answers to my White Hyrdrangea question. I can now sit back and enjoy the late bluish blooms. There's nothing wrong with my babies after all!