Any e-mail FROM Blumenthal TO Hillary could not possibly be considered “classified.” Blumenthal was not a government employee.
If Hillary sent stuff to Blumenthal on the other hand....
Actually, it is not the marking of a document that makes it classified, it is the content. People with security clearances are trained to recognize what is classified. If someone copies a top secret document, removes the cover sheet and markings on it and emails it to someone with a clearance it is still a top secret document. They must report it as a security violation.