In a large bowl, whisk together flour, flaxseed, sugar, baking powder, baking soda, and salt. Add buttermilk, egg, and butter and whisk until just moistened (batter should have some lumps; do not overmix). Fold in blueberries.
Lightly coat a large nonstick skillet or griddle with butter and heat over medium-high. In batches, drop batter by 1/4 cupfuls into skillet. Cook until bubbles appear on top and a few have burst, 2 to 5 minutes. Flip pancakes and cook until golden brown, 2 to 5 minutes. Repeat with more butter and remaining batter (makes about 12), wiping skillet clean between batches and lowering heat if necessary. Serve with butter and syrup.
Freeze cooled pancakes in an airtight container, up to 1 month.