When I had one of these,I just used to take the damaged leaves off.They are probably the old ones anyway,and it looks fine to me..tough as old boots ..:o)

I have experienced this several times in clients gardens, and on some occasions once the leaves start to turn black then the plant can die but not always, this is probably down to wet winters, but yours looks ok compared to some i come across, just nip them old leaves off and it should be ok.