Short answer: toss them in a little flour first, and don’t overload a too-loose batter.
Berries and other add-ins sink when the batter is too thin to hold them, or when their surface is too slick to grip. The easy fix: toss the blueberries in a teaspoon of the recipe’s flour before folding them in — the light coating helps the batter cling and hold them in place. It also helps to have a batter that’s a proper soft-dropping consistency, not runny, and to fold the fruit in last. Frozen berries work too, straight from frozen and floured, so they don’t bleed.
Related:lemon & blueberry cake · glossary:folding,self-raising flour.