You can select any object in Salesforce for sending the email blast that has an email ID field in it.

If you sending the email blast to the recipients of the Contact or Leads object, you can use the Campaign object, add the recipients to it and send an email blast.

And if you are using any other objects like Opportunity, Accounts, etc., for sending the email blast, you should go to the Email Blast tab and select the object and the email field and then send the email blast.