It appears that Dan accuses her of that, perhaps unjustly.
Based on a better Canadian version, it would appear that Serena tries to sabotage DAIR by convincing Dan it was over with Blair.
Blair wants to know every secret Serena has. At first sight this doesn't seem too promising. What are the odds that Serena has a secret more damaging than all the stuff Blair already knows? Then again, because this is Gossip Girl, the odds are pretty good. After all, the producers promised us an "explosion".