Typically, that discoloration is a reaction to stress - an abrupt change in temperature, lack of adequate water, etc. It might also signal a nutrient deficiency - lack of phosphorus. When gardeners alter their soil pH to encourage blue hydrangea flowers, it can throw off the soil chemistry and make the phosphorus unavailable to the plant.

I would run a soil pH test looking fo very acidic conditions. Phosphorus is not well absorbed below 6.0 and hardly around 4.5 (although probably great for blueberries)