Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
It is forbidden for a fasting person to swallow the blood that comes out from his mouth during Wudu. So, he must spit it out. Ibn Qudamah said, 'If one swallows the blood that comes out from his mouth, the vomit, the liquid that comes up from his inside, his fast becomes null since the mouth of a person is considered as an external part of his body.' This is the opinion of al-Hanabilah, al-Shafi’eyah and al-Malikiyah. Al-Ahnaf are of the opinion that if the quantity of saliva is more than blood the fast does not become null; otherwise, the fast becomes null. The first opinion is the correct one, i.e. if he swallows the blood he breaks the fast, if he spits it out his fast is correct.
Allah knows best.