The Collaborative International Dictionary of English v.0.48:

blindside \blindside\, v. t.
   to attack a person from his blind side; metaphorically, to
   give a person an unpleasant suprise. He had completed his
   plan to develop a new office building, but was blindsided by
   the sudden drop in real estate values.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

blindside
    v 1: catch unawares, especially with harmful consequences; "The
         economic downturn blindsided many investors"
    2: attack or hit on or from the side where the attacked person's
       view is obstructed