A blank certificate is a document that certifies something but does not yet contain any specific information. It is typically used in situations where the details of the certification will be filled in at a later date. For example, a blank marriage certificate may be used by a couple who is planning to get married, but who have not yet set a date or location for the ceremony.
Blank certificates can also be used for more formal purposes. For example, a blank stock certificate may be issued to a new shareholder of a company. The certificate will be filled in with the shareholder's name, the number of shares they own, and the date of issue.