It is not permissible to swallow blood coming from the lip or gum because it is naajis (impure)
It is not permissible for the Muslim to swallow blood, whether it is a little or a lot, in Ramadan or at other times, because blood is haraam, unless it is swallowed by mistake or involuntarily, in which case there is nothing wrong with it. Allah, may He be exalted, says (interpretation of the meaning):
“He has forbidden you only the Maytah (dead animals), and blood, and the flesh of swine, and that which is slaughtered as a sacrifice for others than Allaah (or has been slaughtered for idols, on which Allaah’s Name has not been mentioned while slaughtering). But if one is forced by necessity without willful disobedience nor transgressing due limits, then there is no sin on him. Truly, Allaah is Oft-Forgiving, Most Merciful”
The scholars of the Standing Committee for Issuing Fatwas were asked:
Sometimes an individual may get a cut and he licks the blood that is coming out, which results in his swallowing the blood, or his gums bleed, so he swallows the blood that comes out. Is there anything wrong with that?
They replied: It is not permissible to swallow blood deliberately, because it is haraam. Allah, may He be exalted, says (interpretation of the meaning):
“Forbidden to you (for food) are: Al‑Maitah (the dead animals — cattle — beast not slaughtered), blood…”
With regard to swallowing it unintentionally, there is no blame in that case. End quote.
Fataawa al-Lajnah al-Daa’imah, 22/272
And Allah knows best.