Garnet's Player:

Me achin' head! Me burnin' eyes! Now I'm going to have to take out my frustrations on Duras or better yet that rules lawyer Jotha. Where did they go....

I'm not going to get involved with the intricacies of Concurrents. When Garnet levels, she can take a level of Rogue or Cleric, and it will apply fully in Black Plague. You don't need to train or RP. It will keep matters much cleaner and simpler if Garnet takes a level in Rogue, but again this is your choice.