Bring first 3 ingredients and 1/4 teaspoon salt to a boil in a large ovenproof Dutch oven; reduce heat, and simmer 5 minutes.
Combine remaining 1/2 teaspoon salt, flour, baking powder, 1 tablespoon sugar, and nutmeg in a medium bowl; stir in milk just until blended (dough will be thick). Drop dough by tablespoonfuls onto hot blackberry mixture.
Bake at 400° for 35 minutes or until golden. Serve with whipped cream, if desired.