In your Blockchain.info/wallet you can click Import/Export then when it presents you the warning, click "I Understand".
Then on the left click Export Unencrypted.  You can then choose Bitcoin-Qt format.

Now just to recap, from what I gathered from your post:

You created a Blockchain.info/wallet and never imported any keys.  You have received coins to an address(es) in the wallet.  But when you attempt to spend those funds received you get an error saying
  "address doesnt match the private key"

If that is accurate then you might want to open a support ticket:
 - https://blockchain.zendesk.com