First blood RDM implies a particular situation, being, when an innocent gets first blood against a traitor who did nothing.
As everyone knows, traitors are supposed to get first blood, or attempted anyways.
Innocent RDMing innocent is just regular RDM.
The point is though that there wasn't any reason for the kill if it was so soon. Despite whether or not it's on an innocent, it still makes the actual offence worse. At least imo.