The Collaborative International Dictionary of English v.0.48:

Blain \Blain\ (bl[=a]n), n. [OE. blein, bleyn, AS. bl[=e]gen;
   akin to Dan. blegn, D. blein; perh. fr. the same root as E.
   bladder. See Bladder.]
   1. An inflammatory swelling or sore; a bulla, pustule, or
      blister.
      [1913 Webster]

            Blotches and blains must all his flesh emboss.
                                                  --Milton.
      [1913 Webster]

   2. (Far.) A bladder growing on the root of the tongue of a
      horse, against the windpipe, and stopping the breath.
      [1913 Webster]

WordNet (r) 3.0 (2006):

blain
    n 1: an inflammatory swelling or sore