acidity/alkylinity. An acid fertilizer will promote blue flowers and the opposite, pink. My reference are the bottles of fertilizer concentrate in the store. Acid type have a picture of blue hydrangas; the opposite for alkyline. I also remember reading it in a magazine years ago.

I definetely agree,its the type of soil.

I've successfully turned a pink one blue using said concentrate. I don't have acidic soil but wanted a blue one! I expect I'll have to keep doing so in future in order to keep it blue!