A. Aluminum sulfate added to the soil will cause pink hydrangeas to produce blue or purple flowers since it makes the soil acid, says UC master gardener Bill Pierce. You added this amendment too late for its maximum effect. Try adding more this month, which will give the plants plenty of time to react to the acid soil. Once you achieve the desired color level, you will need to add a small amount of the aluminum sulfate every fall to maintain the pH level. You might want to take a pH reading when you get the desired color, as this could be helpful in future years since you will know what pH level is needed for the color you want.
Testing kits are available at nurseries and garden centers.
Your plant's exposure is satisfactory as long as the sun is not burning the leaves. If foliage burning occurs, you will need to attach a shading device to your neighbor's fence. We have a pink dogwood that was planted 40 years ago and has developed a problem. One of the main laterals has been dripping sap out of what looks like a sore. The sap leaves a little color on the tree and so far this is the only spot.