Shalom. If someone ate blood of fish, is this a very bad sin? I only couple of weeks ago, just realized, that fish blood is also prohibited?!? Now, if i am by accident eated blood of fish, is there possibility to repentance? I know that eating blood of flesh, there is punishment that God himself makes.

There is no prohibition against eating the blood of fish and therefore there is no need to salt and “kasher” fish before eating them. It is prohibited to drink fish blood from a cup or other vessel (who would want to?) because someone may think that you are drinking animal blood (=”marit ayin”) [Shulchan Aruch Yoreh Deah 66 9].