Making Black Tea is simple. Pick Tea Leaves from a Tea Bush or buy them in Providence, then place them in a Drying Frame to dry. They will first turn into Yellow Tea Leaves, but leave them in for a few minutes more and they will become Black Tea Leaves. Place one leaf in an otherwise empty pot full of water. The 10L of water will become 9.5L of Black Tea if you've used a full Copper Pot.
When poured into a Barkcup or Flask and equipped in the left hand, the tea can be sipped to acquire a Black Bile regenerative buff that lasts for several minutes. The buff can be increased with more sips. Each sip will add 10 to the Quaffed & Quenched debuff.