Canes, not branches
A blackberry is a permanent root system that throws up temporary canes. A cane grows one year, fruits the next, and then it is finished and should be cut out at the ground. Everything that looks like blackberry pruning is really just that one rule applied on time.
The exception, and it is the one we mostly sell, is a primocane variety like Prime-Ark Freedom. Those fruit on the tips of first-year canes in late summer or autumn and then again lower down on the same canes the following spring, which gives you two crops instead of one. They are worth the slightly more involved pruning.
Tip-pruning is the other habit worth having. Take an inch off a new cane when it reaches about four feet and it branches instead of running, and every branch carries fruit.
Keep four to six canes and no more
A blackberry left to itself turns into a thicket that you cannot pick, cannot get air through, and which gets fungal problems in a Florida summer. Thin each plant to four to six strong canes and cut the rest off at ground level. You will pick more fruit off six canes you can reach than off twenty you cannot.
Give them something to lean on. Two wires at three and five feet between posts is plenty, and a tied-up blackberry is a different plant to pick from than a sprawling one.
Water and picking
Blackberries are shallow-rooted and the fruit is mostly water, so a dry spell while the berries are filling costs you the crop in size and flavour. Mulch and even moisture, rather than an occasional soaking, is what they want.
Pick every day or two once they start. A blackberry is ripe when it is dull black rather than shiny and comes away with no pull at all. Picked a day early it is sour; left a day late the birds have it.