Actually, I didn’t know that blood revenge existed in cases other than accidental killing either (now I see that it’s already in the verses), and I only meant to ask about blood revenge in a case of accidental murder. (And when I asked, what bothered me was the point that there could be a dispute from one extreme to the other about the overall will of God—one says it is obligatory, another says it is forbidden. But I’m dropping that.)

I saw that the Talmud in Makkot brings a Mishnah [and a baraita]: “They hand over to him two Torah scholars, lest he kill him on the way, and they speak to him words appropriate for him, saying to him: Do not treat him as murderers are treated, for this deed came to his hand by error.” It seems that one may indeed try to persuade the blood avenger, but it is forbidden to physically stop him. And seemingly, if there is a moral prohibition on the avenger, then there is a moral obligation on outsiders to help the fugitive and stop the avenger. True, to kill the avenger is forbidden from the standpoint of Jewish law ("do not murder"), but to hold him back or stand in his way does not really involve any halakhic prohibition. They should send bodyguards with him (unarmed), not just Torah scholars. Does the blood avenger’s halakhic permission to kill include a halakhic prohibition on everyone else from interfering?

The very attempt to persuade him shows that the killing is not moral. That is why they try to dissuade him.
Of course, a religious court can also stop him by force. In circumstances where that seems reasonable and moral to them—like nowadays, when the blood avenger’s right is not something socially accepted—they would probably do so. In any case, even if it is not moral, it is the blood avenger’s right, and therefore perhaps they would not use coercion but only persuasion.

The idea is to put pressure on the accidental killer, so that he runs to a city of refuge and does not pose a danger to the public.