In the case of a check payable to John Smith (the payee), a blank endorsement would be the signature of John Smith on the back side of the check without any other words above or below his signature.
A blank endorsement is considered to be risky because the endorser is not restricting the check (or other negotiable instrument). The blank endorsement indicates that whoever is in possession of the endorsed check is considered to be the owner. To avoid such a risk, businesses and individuals should use restrictive endorsements on the checks they receive.