Quite unexpectedly (3,2,3,4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">quite unexpectedly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for out of the blue that I've seen before include "Suddenly" , "From nowhere" , "Quite unexpectedly" , "Entirely unexpectedly" , "Without warning" .)