WordNet (r) 3.0 (2006):

out of the blue
    adv 1: in a way that was not expected; "her brother showed up at
           the wedding out of the blue" [syn: unexpectedly, out
           of the blue]
    adj 1: not anticipated; "unanticipated and disconcerting lines
           of development"- H.W.Glidden; "unforeseen circumstances";
           "a virtue unlooked-for in people so full of energy";
           "like a bolt out of the blue" [syn: unanticipated,
           unforeseen, unlooked-for, out of the blue(p)]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "out of the blue":
   extraordinary, improbable, more than expected, out-of-the-way,
   past expectation, sudden, unannounced, unanticipated, undivined,
   unexpected, unforeseeable, unforeseen, unguessed, unheralded,
   unhoped for, unpredictable, unpredicted, unprepared for,
   without warning