How do I get imported Bitcoin in my blockchain wallet without having to pay more for the private key?
You cannot get the private key. Your money is gone and you will not get it back. Report the crime to your local police.

Google's translation:


  Sie können den privaten Schlüssel nicht abrufen. Dein Geld ist weg und du wirst es nicht zurückbekommen. Melden Sie das Verbrechen Ihrer örtlichen Polizei.


For more information see the answers to Reg: Watch only Address and the answers to the linked questions there. There are numerous other people who have been cheated in the same way.

Do not use the blockchain wallet you created. Any money you put there will be taken by the criminals who tricked you into giving them access to it. Their access cannot be revoked by blockchain or anyone else.