What you describe as "blackballing" is not, generally, illegal unless there is a hidden agenda of some sort.Well, if I were handling Royal Blood or Foo Fighters, or an number of the other artists who are being screwed over (or have been screwed over), I would be exploring recourse because a blackballing is definitely going on.
An artist that did not show for an interview or appearance can be taken off the playlist, for example. In fact, the simple condition of the PD not believing in a song or an act is a valid reason not to play them. There is no immutable standard that requires unconditional airplay.
And often the "blackballing" is quite simply the result of a station music test that showed that the artist had too many negative scores to be playable, even if some listeners loved them or the individual songs.
What you perceive as "blackballing" is most likely just good, well researched and well reasoned programming.