Q.blackberry bushes
Why would some berries look like they did not mature and turned brown?

Certified GKH Gardening Expert
Brown blackberries on the plant can be the result of diseases like anthracnose:
https://extension.psu.edu/plants/tree-fruit/news/2013/blackberry-disorders-that-affect-fruit-quality
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberries-not-fruiting.htm
Also, check this blackberry diagnostic tool to see what matches the symptoms your plants are showing:
https://projects.ncsu.edu/project/berries/diagnostic_tool/index.php