As an experiment, you might try keeping one of your plants in the garage in a dimly lit place until well into spring so it has delayed development, later blooming and fruiting. Keeping the pot cool would be most effective.

How late in the year do you want to be harvesting?

Looking for blackberries anytime after triple crowns. Immediately after or much later is fine. Just looking for a variety to spread out the harvest.

My Prime Ark 45 blackberry plants are starting to ripen now and look to produce until frost. They also produce an early crop in late June in 6a. Flavor is decent but not as sweet as TCs but the berries can be larger.