Check out this graphical, colorful study of movie posters since 1914... but note the dramatic increase in blue-hued ones. Since when did blue become a dominant shade? The original blog post by the artist/creator quotes a graphic designer explaining:
"Perhaps the colors say less about how movie posters' colors as a whole and color trends, than they do about how genres of movies have evolved. For example, there are more action/thriller/sci-fi [films] than there were 50-70 years ago, which might have something to do with the increase in darker, more 'masculine' shades."