The Collaborative International Dictionary of English v.0.48:

Blinder \Blind"er\, n.
   1. One who, or that which, blinds.
      [1913 Webster]

   2. (Saddlery) One of the leather screens on a bridle, to
      hinder a horse from seeing objects at the side; a blinker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

blinder
    n 1: blind consisting of a leather eyepatch sewn to the side of
         the halter that prevents a horse from seeing something on
         either side [syn: winker, blinker, blinder]