Brooklyn Mitts, having the double layer of lace at the top, definitely require blocking. This is how I block mitts. My mitt blockers are pieces of cardboard cut to the dimensions below, then wrapped in several layers of plastic wrap for water proofing.
Then I GENTLY squeeze out the water and pin onto my mitt blockers. You will need to pin on both sides, opening up the lace scallops to shape. Stuff the thumbs with a rolled up piece of tissue. Then I hang the whole set up on a clothes line to dry.
Voilå...perfectly blocked mitts!