From what I have experienced and read your Queens Wrath rep must be at least lvl 3 at the time you finish to receive BG from Petra. My warlock is rank 4 and got it but my rank 2 hunter did not. I dunno if you can get it after the fact if you meet the rank requirement later. EDIT: I can confirm now that you can get after the fact finally got my hunter to rank 3 and it was waiting for along with my rank up package.