Biblical speaking human blood is not prohibited, in the virtue of thora law, it is only prohibited by the virtue of rabbinic law and the reason is because you may confuse it with animal blood. The rabbinic tradition is: When you are bleeding internally in your mouth, you dont have to spit the blood out. If you bleed in the finger, you are not supposed to suck on your finger, once the blood leaves your mouth you should not put it back into your body. But that is not a biblical but only a rabbinic prohibition.
The Shulchan Aruch (Yoreh De'ah 66,10) states that human blood, after it has left the body, is forbidden. This is not because the human blood itself is forbidden to us from the Torah, but rather because someone might think mistakenly that it was non-human blood and therefore forbidden (this type of law is called "marit aiyin"). So, if one bites an apple and finds that blood has come out of one's gums onto the apple, the blood spots must be removed from the apple before taking the next bite. However, continues the Shulchan Aruch, blood inside one's mouth is allowed, and so if one has bit their cheek, or has bleeding gums, the blood inside the mouth may be swallowed, and one does not need to spit it out.
There is some discussion amongst the Rabbis as to the definition of this prohibition. Is it only permitted when the blood is flowing inside one's mouth because it never "came out" as it were? This seems to be Rashi's understanding. If so, blood on a cut, even though it is clearly human blood and permissible, would still fall under the Rabbinic prohibition, as all external blood was forbidden. Or is it always permitted whenever it is clearly human blood, as when it is dripping from a cut finger? This is Tosafot's understanding. If so, the blood from a cut finger is readily identified as human blood, and is allowed. Many opinions rule leniently, and allow one to suck on a cut finger, whilst there are some stricter opinions who forbid it, or only allow it if the blood is inside the wound and will be sucked out, but not if it has already flowed out onto the skin of the finger. (See Nishmat Avraham on the Shulchan Aruch ibid).
From an answer to the Question "Blood from wound -kosher?"
Why is certain animal blood forbidden after kosher slaugther? Because it served as korban under the alter in the temple. Blood of fish is biblically allowed to be eaten. There is blood that is allowed. The same is certainly the case with the blood of grasshoppers in some traditions.
The point of interest: not all animal blood has been biblically but only that which serves as korban according to biblical law. .
Blood - There is no prohibition against eating fish blood, other than the fact that people may think that a
person is eating prohibited blood in which case a sign must be posted indicating that the blood is fish
blood. Ritual slaughter is not required.
Human blood which inadvertently got mixed with food (such as blood from a cut that dripped into food) may be consumed as long as no bloody redness is visible. This is true even if there is more blood than food in the mixture. If redness is visible, then the food may not be eaten, even if the volume of the food is sixty times greater than that of the blood(24). If blood gets mixed into food, additional food may be added to the mixture in order to make the blood invisible(25).
Thats a treasure of the bible, that there is no ban in Leviticus of consumation of human blood but only on animal blood to find. Hence even a saving life with "nourishing" transfusion would be also permissable. If it were garantueed that it is not mixed with animal blood and that human blood contains no demons than what speaks against transfusion? In anticity they believed that if you consume blood of an animal that you would become animalistic. It would somehow diminish your humanity/sensititvity.
Lets eat fishblood and grasshoppers blood withouth fear of loosing eternal blessings so!