Can you post pictures? Does it easily come off? Most of the time, fungal infections start on the leaves and can propagate to stems. Really bad infections of anthracnose can be a problem. But other times, the issue is just leaf spots. Controlling the excessive humidity should be priority one. By viewing what you are seeing, someone might be able to identify THE problem. Leaf spots sometimes automatically self correct once summer weather patterns start (low humidity; high temperatures). In the meantime, make sure that you do not do overhead watering. Instead, water the soil early in the mornings. Also, water when the soil feels dry if you insert a finger to a depth of 4"; you can also water if it feels almost dry. Dispose in the trash all plant debris found under the shrub.

It's never too late to add data and experiences. Every hydrangea plant is different in one way or another, and there is generally a terrible lack of user experience and feedback with these plants! To make matters more confusing, hydrangeas are like cell phones - new ones come out when people aren't even done testing out the existing releases! Out of curiosity, what's your hardiness zone in MD? Parts like around Deep Creek go all the way down to 5a... Also word is from PW that Moonlight is going to be dropped entirely (an official rep posted that in THIS forum). They claim to favor Big Easy instead, which is a fine plant but nothing spectacular. I'd actually recommend Rhythmic Blue, but you'll only get deep colors (not like anyone's complaining). Blue Jangles and Rave are a bit newer and largely untested. As for the other older Cityline cousins Berlin, Venice, Vienna, etc. not a whole lot has been posted about them. I have Venice and it didn't do squat after '13 and '14 winters so it's been in a pot awaiting its ultimate fate. Just gave five "axes" (Charm/Compacta, Merritt's Supreme, Mariesii Variegata/Tricolor, Snow Storm (White King), and F&E Original (Early Sensation) to my neighbor the other day. :) I'm almost inclined to recommend people try Pink Shira in lieu of Moonlight. I'm not sure what my one neighbor has, but it's been covered in 4-5" pink blooms yearly for four straight years, so I have a distinct feeling it is Shira. I found myself one this year so we'll see how it does in the Great Outdoors!

I would not intentionally plant a bigleaf hydrangea in full sun in any zone. Regardless of what the plant tags may say, these are essentially woodland plants and need a part or dappled shade environment. Morning sun & afternoon shade is fine. Even in my very mild and far northerly zone 8 location - where the sun is not very intense and it never gets too hot - bigleaf hydrangeas in full sun wilt on a daily basis.

As the blooms age, they start adding greenish tones. After several weeks of this, you get another change where the blooms develop pink/red/purple splotches like that. Finally, the blooms start to brown out. Some varieties produce a lot of this and others less. Try to water the soil and never the leaves so you do not get leaf spots. Use soaker hoses or drip irrigation. If using a sprinkler, make the station water close to sunrise so the leaves do not stay wet for long periods of time.Infected leaves should be discarded in the trash when they fall down. Spent blooms and all plant debris from plants with leaf spots (a fungal infection) should also be discarded asap to minimize repeats of this problem at the end of the growing season.