Darkwood is a material added by the Natura mod. Spawning only in the Nether, the Darkwood Tree is dark blue in color, with dark blue leaves that have green and white flowers. When chopped down, the Darkwood Tree drops Darkwood Saplings and sometimes Potash Apples. It is around the size of an average oak tree.
Darkwood can be turned into Darkwood Planks and Darkwood Sticks, and the sticks can be used to craft a Darkwood Bow.
The wood can be turned into wooden planks which can make sticks, darkwood door and darkwood tools (Sword, Pickaxe, etc.)
As stated above, potash apples can sometimes drop from the tree. But an easier way to farm them is to break the white-flowered leaves, since they have a high chance of dropping the apples instead of the green ones, which will gradually turn into the white leaves and produce apples.