A blind stamp is a mark pressed into a book's board or paper without ink, so it shows only as an impression. On books it most often refers to the small mark on the back board of American book club copies, but publishers, binders and libraries used blind stamps too.
How to spot it
Tilt the back board of the book under a light and look at the lower corner near the spine. A book club blind stamp is often a small dot, circle, square, maple leaf or other shape. Library blind stamps appear on title pages and plates. Run a finger gently over the surface: an impression can be felt even when it is hard to see.
What it does to value
A book club blind stamp identifies a book club edition, which is worth far less than the trade first edition it resembles. A library blind stamp identifies an ex-library copy, also a significant fault. A publisher's or binder's blind stamp is usually a neutral feature of the binding.
Our book club editions guide shows where clubs placed their marks, and ex-library books covers library stamps.