הלכה על כריתות 41:25

From the laws of the commandment is that which they, may their memory be blessed, said (Keritot 21a) that one is not liable for excision on account of blood, with the blood of fish, locusts, disgusting animals and creeping animals, nor the blood of people. And therefore they said that the blood of fish and locusts is permissible, that it is permissible to eat their blood - and even if he put it into a vessel. And that is when it is recognizable to all that it is the blood of fish. And, like they said in the Gemara (Keritot 21b), that there be a few of the scales of the fish in the vessel. But the blood of forbidden fish is forbidden, since it is like the milk of an impure beast, which is forbidden from the principle that is in our hands - food that comes out of the impure is impure. As we see that the Torah forbade the egg of an ostrich, since it came out from the impure (ostrich).