Sterilize sharp pruning shears with alcohol and rinse with clean water.
Prune blackberry bushes in the early spring by cutting the top of the blueberry canes down so that the cane is no longer than two feet. For shorter canes, simply cut off the tips of the cane.
Prune in the summertime after the blackberry bush is done producing fruit for the season. Since blackberries only produce fruit on each cane once, removing any canes that produced fruit this year will encourage the plant to grow new canes--and therefore more fruit--next year. Use your sterilized pruning shears to cut off any canes that produced fruit this year at their bases.
Remove any dead or diseased canes from their base on the bush in both early spring and late summer. This prevents the spread of disease to other canes.