I used frozen blueberries and buttermilk powder. My buttermilk powder says to use 1 cup water + 1/4 cup powder for 1 cup buttermilk but this makes the batter too liquidy. Reducing the water to 3/4 cup would have been better.
I didn't find that these muffins to be as awesome as some of the other commenters, pretty average, really.
In case anyone else only has pre-ground flaxseed, I found that 1/5 cup flaxseed yields about 6 Tablespoons ground flax seed.