While the donuts are cooking, make the blackberry glaze. Add the blackberries and lemon juice to a food processor and pulse until pureed. If you'd like the blackberry glaze de-seeded, strain the puree through a fine mesh sieve
into a small pot. Use the back of a large spoon or spatula to press the puree through the sieve. If not de-seeding the puree, transfer the puree to a small pot.