Blotchy blueberries

[Suemarie]

My blueberry bush is several years old, in a large pot with ericaceous compost, and fed appropriately. I've only ever given it tap water as there's no room in my yard for a water butt though it's never affected the crop. The bush is laden with more fruit than ever but about 25% have a hard blotch on them (see photo). Could someone please tell me what is causing it and if there's a cure?
Many thanks

[burnie]

Firstly welcome and secondly sorry, never grown them and haven't seen anything like that before. Hopefully someone else will be along soon.