How long have you been using 2FA?
Since 2FA enabled it means that your Private key were compromised, not your Blockchain Account.
If 2FA were turned on recently, that means the thief has access to your account for a long time and he has dumped your Private Key to another wallet.
If you got 2FA since you've registered on Blockchain, it means either you have clicked on Phishing link by accident or Rogue Browser Extension that steals your 2FA key.

I hope you don't lose much, just use this opportunity as a lesson and start using Desktop or Hardware Based Wallet, much secure