I'm confused here. It is written "You may choose either Bluff or Diplomacy as a class skill, and gain a +2 bonus on such skill checks." So, how it works with regular class skills?
RAW, it means that you add Bluff or Diplomacy -your choice- to your class skills (check the corresponding box), and gain a +2 (to write in the Misc. Mod column of your sheet) to the skill. If both are already class skills, well, that's something the trait won't bring you. You still get the +2 though. Since it is considered a class skill, you also get the corresponding +3 bonus if you put ranks in it.