The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) discusses the status of various types of blood with respect to the Torah prohibition against ingesting blood (listen to audio recording for precise citation). He writes that the blood of animals and birds is forbidden for consumption by force of Torah law, but the blood of fish and grasshoppers is forbidden only by force of Rabbinic enactment. Torah law permits the consumption of the blood of fish and grasshoppers, but the Sages forbade ingesting this blood due to the concern of "Mar’it Ha’ayin," meaning, that onlookers will mistakenly assume that one is eating animal blood. If the scales of the fish are on the table, such that onlookers will immediately realize that the person is eating the blood of fish, then the blood is permissible.
This applies to human blood, as well. According to Torah law, there is no prohibition against ingesting human blood, but the Sages forbade ingesting human blood out of the concern of "Mar’it Ha’ayin." However, this applies only to "Dam She’piresh," meaning, blood that has left the person’s body. For example, if a person’s tooth begins to bleed while he is eating a piece of bread, he may not eat the blood on the bread; he must cut off the part of the bread that has blood on it, and he may then eat the rest of the bread. However, he is not required to expectorate the blood in his mouth. Since that blood never left his body, and has remained inside his mouth, it does not fall under the category of "Dam She’piresh" and is permissible for consumption. This would also apply in a case where one’s teeth or gums bleed as he brushes his teeth. This blood may be swallowed, since it never left his mouth.
The Ben Ish Hai also writes that if a person’s finger is bleeding, he may place the finger inside his mouth and suck the wound to help stop the bleeding. Since it is readily obvious that the blood originates from his wound, and not from an animal, there is no concern for "Mar’it Ha’ayin" and the blood is therefore permissible. However, if the bleeding has stopped but some blood remains on the finger, he may not lick the blood off his finger.
Summary: If a person’s mouth began to bleed as he was eating, he may not eat the part of the food that has blood on it, but he may swallow the blood that had never left his mouth. Similarly, if a person’s mouth began to bleed as he was brushing, he may swallow the blood, provided that the blood had never left his mouth.
