I think the maggots you seen in ripened blackberries are of fruit flies (Drosophila sp.). The adult female flies lay eggs just under the skin of fruits. The maggots develop and feed inside the fruit, causing the flesh to turn brown and soft with sunken areas that can exude fluid on the surface. This damage also act as entry site for fungal and bacterial pathogens. After maturation, this maggots fall out and pupate in soil.

Management:
1. Collect the fallen fruits and destroy them (you can place this fruits in plastic bags, seal it and place it in trash).
2. To avoid oviposition on fruits cover the trees or canes with fine nets. Netting should be done before fruits start ripening. The recommended mess size is 0.98 mm.
3. Also you can harvest fruits early to reduce flies damage (harvest as soon as the fruit ripens).
4. You can use traps to monitor fruit flies. Traps can be purchased in the market or you can prepare one. Take plastic container with lids (one quarts yogurt container is fine). Drill holes (10 to 16 holes) that are 3/16-inch in diameter around the upper side of the container. Add 1 to 2 inch of pure apple cider vinegar (not flavored one) and a drop of unscented liquid dishwashing soap into the container. Hang the container in shade near berry trees before fruits ripening and check the traps frequently for flies. You have to change the vinegar every week.
5. If infestation is severe you can spray suitable insecticide.