Even not considering the "broad, metaphorical meaning" and focusing on the literal meaning of blood libel which led to pogroms and killings, the term 'blood libel' is accurate because it seems that those who are libeling conservatives and conservative broadcasters would not be necessarily unhappy if they incited violence against the right.
Should we only use the term 'blood libel' after the murders have happened, or should we use it if we believe we have started on a path to violence?
It we say some actions are fascistic, it does not mean we are living in a fascist society but if government keeps doing fascistic things, it won't be long before society is fascist.
Likewise, the libel may not now be calling for literal blood but it certainly could put us on that path.
"Blood libel" seems an accurate term to use to describe a false accusation of an offense sufficiently heinous that it would provoke bloodshed against those so libeled.