Charcoal dust was added among other dyes to the mod as an alternative to vanilla methods to obtain colours.
First burn some kind of wood (not planks!) in a normal furnace or a Forge (except orc forge) to get charcoal. Then put a piece of charcoal in a normal crafting table or your personal crafting grid to obtain a piece of charcoal dust.
You can use the dust in the same way as you use vanilla ink sac, a more specific and useful way of using it is in Blackroot Vale Armour.