The blood pen is used for contracts where one's integrity is paramount. The process of withdrawing and writing in one's own blood signifies one's intent and commitment, and reflects on the inevitable pain, difficulty, and sacrifice necessary in fulfilling one's word.
The ideal instance for the blood pen is signing a peace treaty. The pen signifies the end of bloodshed and is the literal final drop. Decision and action unified.
Blood contains your genetic code. In blood, your signature not only represents you, it is you.