There are visual indicators for blackberry maturity, but the best way is always to eat one. When the shine on a blackberry begins to dull, that's usually as ripe as you want it. Dead ripe berries also separate from the panicle with much less pulling force. Beyond that, the berry can either fall to the ground or come apart in your hand when you pick it. Over ripe berries also attract insects and can begin to mildew. Better to pick them a little sooner and add sugar than to see them end up in the dirt.