Yes, even after blueberries have been frozen, they can provide you with excellent antioxidant support. It's important, of course, for your blueberries to be high quality before they are frozen, and for this reason, I recommend organically grown blueberries. Always avoid blueberries that are dull in color.
There's good research showing very little loss of the anthocyanins – the unique antioxidant in blueberries – after freezing of healthy, ripe berries.
For other antioxidant nutrients, the loss due to freezing varies. The berry will lose some vitamin E, lutein, zeaxanthin, and beta-carotene with freezing. It will also lose a substantial amount of Vitamin C. So though fresh is best, you will still gain benefit from frozen blueberries!