It is a distinguishing mark for the artist or owner.
Blind stamps are usually quite unobtrusive, although careful inspection may reveal quite a decorative image. They are occasionally seen on the covers and spines of books, especially older ones.
Here are some photographs of blind stamps.
Blind stamp on an antique print by William Paxton. It is an image of an elephant and was likely put there by the owner of the print.
Blind stamp on an old book.