The blackroot is a herb that grows only in the Cloisterwood on Reaper's Coast. The root is crucial for the quest Powerful Awakening. A single blackroot can be found in the cupboard, next to a table where Meistr Siva sits, within the Meistr's House cellar.

How do you do a Blackroot ritual?

Once in the basement, she will ask you to perform a ritual. The ingredients are found in the cupboard behind her. Once you have them, use the Obsidian Knife on your main character. Then select combine and mix the bloodied knife with the bowl and blackroot.

Where is Cloisterwood?

Cloisterwood is a sub-area of Reaper's Coast in Divinity Original Sin: 2.