Thanks for the picture. Did the near white leaves fall off? Iron deficiency is common in hydrangeas and can turn leaves almost white which is why I was going along that train of thought from the initial description, but it doesn't look like iron chlorosis and nor is it likely either given that the plant is flowering blue.
Hard to see a pattern but it looks as though the leaves are yellowing starting at their tips, and that it's the older leaves on a stem that are most affected? With nitrogen deficiency the older leaves would be affected first and the veins would not remain green. Looking at the one branch that is sticking up above the rest towards top right, there does seem to more or less be a progression to paler going down the stem which would be consistent with nitrogen deficiency.
There is a conifer next to the hydrangea that might be competing for nutrients and moisture. Is there any possibility the soil is on the dry side there because of the deck (?) and the conifer?
