Freeze them for sure.
You can use them in so many different ways
pies, crisps, crumbles, cobblers, muffins, cheesecake topping, jam, jellies, smoothies, pancakes/waffles (in or as a topping), with green salads, mixed fruit salads, with yogurt .................. the list is endless!
Really, anywhere that you use other berries, just substitute.
I bet there are even blackberry wine recipes, if that is of interest to you.