Hypericum help

Hi all
I have a Hypericum (rose of sharon) which has grown very well from cuttings from a well established shrub.I have noticed that the buds which have formed are black and almost burnt-like.The rest of the shrub appears healthy.The shrub is in a partial shady position in the garden.Does anyone have any ideas what the problem might be?
Thanks

 

Sounds like frost or wind damage. It could be insect damage, get a magnifying glass to it.

Does anybody know how many shrubs are called "rose of sharon". It looks like a default name for any shrub.