The past 4 years, I've had perfectly deep blue hydrangeas. Last fall, I planted some Spring bulbs next to the bush, and of course fed the bulbs with high phosphorus bulb food. Now my hydrangea is coming in pink on the side where the bulbs are. With a bit of Googling I learned that higher phosphorus levels prevent hydrangeas from taking up aluminum from the soil, thus preventing them from forming their blue color.

So that's my tip. If you want blue hydrangeas, don't get bulb food or bone meal near them. Of course if you WANT them pink, well there you go.