R. Chanina b. Gamliel says: This is blood from a living animal.
(Ibid. 12:25) "You shall not eat it": to include (as forbidden, eating) meat with milk. (Why is a verse needed for this?) Does it not follow a fortiori, viz.: If neveilah (carcass), which is permitted to be cooked, is forbidden to be eaten, then meat with milk, which is forbidden to be cooked, how much more so should it be forbidden to be eaten! — (No.) this is refuted by mixed seeds (klai hakerem), which, although they are forbidden to be sown, are permitted to be eaten. Do not wonder, then, if meat with milk, even though it is forbidden to be cooked, would be permitted to be eaten. It must, therefore, be written "You shall not eat it" — to include (as forbidden to be eaten) meat with milk.