Plain English Meaning
A small box that measures your altitude and feeds it to your transponder so ATC can see it on radar. It is called "blind" because the pilot cannot see what it is reporting -- it only talks to the transponder, not to you.
Full Definition
An altitude-encoding device that senses pressure altitude and sends that data to the transponder for transmission to ATC, but does not display the altitude to the pilot. It works in the background to support Mode C altitude reporting without providing a visible readout in the cockpit.
Context Anchor
Seen in Mode C altitude-reporting discussions, transponder checks, and instrument-panel equipment descriptions.
Derivation
"Blind" here means "without a display" -- the device does its job unseen, much like a "blind" valve or fitting in engineering means one with no opening or readout. The pilot never reads the directly; only the transponder and ATC see its output.