"Collect blood fruits" is a mission objective in the quest Denial, Anger, Initiative in Tiny Tina's Assault on Dragon Keep DLC
Collect the Blood Fruit from the Blood Trees growing around the Blood Tree Camp
Note: When you pick the first Blood Fruit, the tree will turn into a hostile Treant. Do NOT kill it if you want to complete the next optional objective that Tina gives you.
Tip: If the Orcs around the camp are too hard, just avoid them and run around collecting the fruits. You can also lure the Treeants towards the Orcs and they will fight each other.