Blood Rage specifically states that the unit's HP is zero. If the damage can't slay then it does nothing if Blood Rage is active.
Lemme rephrase: since Blood Rage triggers on going to zero, and the unit is specifically not slain, can abilities like Plague reduce a unit with Blood Rage from 1HP to 0HP? My intuition is that they cannot, but if you were to ask me why I couldn't give you a great answer.
(but yeah these abilities obviously can't slay a unit that has reached zero hp, activated Blood Rage, and is now at zero hp but not slain yet, and apologize for not making my question clearer)