As you take your casual stroll through the Erebus-or panicked sprint, if you're one of those people-you'll see a large number of hanging vines around the place. Those are Dark Fruit vines.
By breaking a piece of vine with fruit on it, it'll drop up to three Dark Fruit. These fruits restore 2 hunger points (1 hunger icon) per fruit. They are classed as a low-tier food.
If you break a piece of vine with yellow bulbs on it, it'll drop up to 2 Dark Fruit Seeds. These seeds can be planted on the underside of virtually any solid block, to grow vines that produce more seeds and Dark Fruit. Be wary; if you break above a vine with Dark Fruit or seeds on it, it will not drop those items.
Dark Fruit and its seeds serve no other purpose beyond making a smoothie with the ErebusCo. Smoothie-matic 2000.