Q.Bluberry brown marks
Why are my blueberries getting marks that are hard when blueberry is opened?
From just this image; it can be hard to tell. You will want to do careful inspection of the plants; leaves, stems of the Blueberries.
I suspect this is an Anthracnose: which can be treated by use of fungicides.
I've listed some articles to help you with diagnosis.
You also could take some samples to your County Extension Office, they may have more information on regional issues in fruit crops.
https://ag.umass.edu/fruit/ne-small-fruit-management-guide/highbush-blueberries/diseases
https://www.gardeningknowhow.com/edible/fruits/blueberries/treating-blueberry-mummy-berry.htm
https://www.gardeningknowhow.com/edible/fruits/blueberries/treating-blueberry-botrytis-blight.htm
https://www.gardeningknowhow.com/extension-search
