we live in Astoria oregon and moved into a house with three well established heavy bearing blueberry bushes of unknown variety. Some of the ripe berries have a hard brown spot at the flower end. I am sending a picture of the berries and the leaves of the bushes. the bushes are very healthy and I see no evidence of leaf or stem problems. I'm thinking these spots are some sort of residual leaf part.
Through the years the calcium in the soil has probably become somewhat depleted. Blueberries lack a deep tap root that can allow better access to nutrients deeper in the soil. Their shallow root system consists of many small branches which can extract the available nutrients nearer the soil surface more rapidly, so they may have exhausted the calcium available to them. This may cause the browning of the blossom end on your berries.
Agricultural gypsum applied in the fall and spring can correct the deficiency. Agricultural gypsum contains about 25% actual calcium so a pound of gypsum per plant replaces about 4ounces of depleted calcium and is quickly available to the plant roots. If you buy a blueberry nutrient at your local nursery check the label to be sure it contains calcium.
Thank you for your question.