Answer: Blackberries do best on a rich soil well amended with organic matter and with a steady level of moisture. It is possible that the surrounding trees have sent roots into your bramble patch and the blackberries are suffering from that in addition to insufficient sunlight. Top dressing with compost or well rotted manure, and maintaining a good layer of organic mulch might help if that is the problem. You might also consider some tree work to allow more light to reach the berries.
Blackberries require a regular pruning regime; after harvest both the bearing canes and weak new ones should be removed; canes should be pinched in preparation for winter, and then in spring the laterals should be cut back by about half.
Finally, it is possible that you are growing a variety not quite suited to your climate. You might wish to check with your County Extension (tel. 677-8223) to see which are most highly recommended for your area. You might also, after four years, consider retesting your soil. They can help you with the tests and interpreting the results as well.
Q&A Library Searching Tips