All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If a fasting person is bleeding in his mouth, he is obliged to spit it out and it is not permissible for him to swallow it.
Imaam Ibn Qudaamah [from the Hanbali school] said: 'If a fasting person is bleeding in his mouth or has vomit, and he swallows it, his fast is broken even if it is a small amount, because the ruling that applies to other apparent things apply to the mouth as well. In principle, anything that goes through the mouth breaks the fast, with the exception of saliva as it is difficult to avoid it.' The Shaafi'ee and Maaliki schools are of the same view as the Hanbali school.
Therefore, if you deliberately swallowed some blood, then you have to make up the days in which you deliberately swallowed the blood. However, if the blood was abundant and entered your stomach without you deliberately swallowing it, then this does not invalidate your fast and you do not have to make it up as you did not intend to swallow it.
Allaah knows best.