Blueberry blotch

Hi. 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