The phrase "blind stamp" refers to a mark or impression that is made on a piece of paper or material without using any ink, so it is not visible to the eye.
Sentences with «blind stamp»
Artist's initials in pencil lower left margin of sheet beneath image; inscribed «AP 14/14» lower left margin of sheet; printer blind stamp lower right edge of sheet (lamodern.com)
Initialed and dated in pencil lower right margin beneath image; printer blind stamp lower right edge of sheet; edition lower left margin of sheet; retains a John Berggruen Gallery label verso (lamodern.com)