Good Morning Fiona,

Thanks for reaching out! I got a response from Ann Hazelrigg, the director of the Vermont Plant Diagnostic Clinic.

Here's what she had to say:

The leaf edge dieback/death can indicate a problem lower in the plant such as mechanical injury, root rot, etc. I don't know of a foliar blight in hydrangea that would do this. 

You may want to clip out the dead portions and see if new growth starts from the base. If not, it is likely the roots have died. It may have been too wet with the run off and saturated soils,  although hydrangeas usually can survive with lots of water.

You will not be moving the disease around if you get rid of it but i would not put anything back that does not like wet feet.

I also love this circular from Virginia Tech all about Hydrangeas! https://chesapeake.ext.vt.edu/content/dam/chesapeake_ext_vt_edu/files/pruning-hydrangeas.pdf

Hope this helps! Happy gardening!