The cartridges are refillable by pushing an open paper clip into the hole and releasing the cup that holds the juice. There's a vid out on the web showing how to do this. Unfortunately there's no cap on the cartridge to seal it after a refill as you can do with the 510 cartridges.
My way around this was to buy four extra atomizers for $19.90 shipped, and then use them as "caps." Refilled blu carts with atomizers attached still fit perfectly into the cart slots in the pack (atty-side up), so you can have one fully-assembled blu, one extra battery, and five refilled carts-plus-atomizers in the pack. I've been doing this for two weeks, and haven't had any leakage or other problems yet. In fact, vapor production has gotten way better, because none of the atomizers gets overused since I'm cycling through them every time I change carts, and they all stay moist because they're all wicking liquid all the time.