Dark Wildwood Leaves grow on large and lush vine-covered Wildwood-trees in Jungle-biomes. Wildwood Flowers are usually embedded in these leaves, most often the kind of flowers that does not glow in darkness, but will turn into the glowing kind if harvested. You can more often find another kind of leaves on the same Wildwood-trees; the ordinary brighter colored Wildwood Leaves.
Blocks of Dark Wildwood Leaves can be harvested/pulled without needing any Power Cells equipped. Wood Mining Cells will lose durability by harvesting Leaves or any other soft material. However if you use Stone Mining Cells or better to pull leaves, these (stronger) Power Cells will not lose durability by that.
In Creativerse tree-leaves do not decay / vanish by themselves currently (as of R27 in January 2016) when you "cut" the stem of the tree nor if you take away all the Wood-blocks that trees are made of. Instead the blocks of leaves (as well as all Flower-blocks, Beeswax, Vines, etc.) will prevail and float in the air until you take them down manually or will use Excavators on them to collect at least half of them.
You can of course also throw Explosives at leaves and/or burn them. As leaves are flammable, and burning them (by throwing Fire Bombs or placing torches under blocks of Wood or Leaves) might work overly "well" in warm to hot environments like Jungles where Wildwood-trees usually grow. So beware of forest fires that can burn down a whole Jungle as well as all other kinds of flammable blocks and materials in reach of the flames, including wooden buildings. Fire cannot be extinguished currently (as of R27 in January 2016), you can only stop it from spready by cutting aisles.
Taking/crafting Shredded Leaves will unlock the recipe for three types of colored carpets. Shredded Leaves burn easierly than fresh leaves when being placed (even on snow) and can be used as a fuse for Explosives very well.
Dark Wildwood Leaves can be placed as solid blocks for decoration and can also be used for building; however they can make creatures spawn, like green Leafies and Chizzards during the day, plus Night Leafis as well as Night Chizzards in darkness at night.
Dark Wildwood Leaves can be used as Fuel for the Forge, however 4 blocks of leaves are needed and used up for each melting-/hardening-process. Also each melting-/hardening process in the forge will take quite long, as simple leaves make weak fuel.
Dark Wildwood Leaves can be corrupted into Corrupted Leaves, for example by using Corrupt Bombs on them. By that they will turn into better Fuel, but they have to be picked up using either Diamond Mining Cells or Lumite Mining Cells, which will reduce the durability of these Power Cells quite a bit (just like pulling all other kinds of corrupted blocks). If you purify these corrupted leaves, they will turn into Cragwood Leaves, even if they were Dark Wildwood Leaves before being corrupted.
You can lay out Dark Wildwood Leaves to make the bright glowing kind of Wildwood Flowers grow on them (different to the dark non-glowing kind of Wildwood Flowers that grow on Dark Wildwood Leaves "naturally" on Wildwood-trees), however that will take quite some time (a few RL-days for each new block of Wildwood Flowers). Currently (R27 in January 2016) you cannot grow Wildwood-trees from any Saplings yet.