Blackberries seem to ripen in clusters, with whole clusters being ready at the same time. Raspberries, which peaked in June, are smaller than blackberries and often only have one ripe berry on the tip of a cluster. Blackberries are ripe when they are plump and very dark purple, almost black. They should come off the stem easily by just rolling them between your thumb and index finger and into the basket. If you have to tug to get them off, they usually aren't as sweet, so I don't bother with them.
Blackberries range in size from having only a few seeds to being as big as strawberries.
Besides wild, thorny blackberries, there are also thornless cultivars. Some say these aren't as sweet, but the ones I've tasted were sweet, juicy treats with just a slightly different taste. The best way to enjoy blackberries is probably right off the vine. It's instant reward for your effort, and a nice refreshment on hot days. It's also nice to have a piece of blackberry cobbler or pie just after they're picked.