The phrase "blank form" refers to a paper or document that doesn't contain any information or is not filled out yet. It is a template that is ready to be completed with the necessary details or data. Full definition
Never sign blank forms or leave details for the broker to fill in later. (moneysmart.gov.au)
The middle blank forms the tunnel — the actual backbone of the passenger cell. (topspeed.com)
We often hear horror stories about clients who are asked to sign blank forms and are simply given assurances that their immigration matters will be taken care of by someone else. (southasianpost.com)