are they getting enough water?? hydrangea are very thirsty plants to begin with and pots dry out fairly quickly...amount of sun may also be an issue...they generally prefer shady conditions. if sun is unavoidable, it should be morning sun rather than late afternoon sun (which is much more intense/hotter).
should the stems have brown/black spots on them? the black/brown are all over the leaves. Some of the lower leaves are turning yellow. could they have gotten to much water ,the pots were under the roof drip line.
i see some spots on the stems also. mine are under the overhang from the porch...so, they do get some of the overflow from the gutters (as yours do). i also see some of the leaves closest to the bottom get yellow. i never worried about the yellowish leaves since they're at the bottom and don't get any sun - just always figured it was due to being in complete shade all the time once the upper part grew in. i don't think you have anything to worry about - unless the containers do not have drainage. that would definitely be a problem. can you post some pics?? it basically sounds like what you have happening is the same that goes on with mine...i'd like to see pics though, if you can do it. just to be sure there isn't something going on that needs to be dealt with.
