mplayer-skin-blue provides the package mplayer-skin, meaning that mplayer-skin is a dependency of mplayer-skin-blue. Listing mplayer-skin as a dependency does not provide mplayer-skin-blue. I am not sure how Debian apt handles this.
Usually Debian meta packages like xorg are processed as intended, but as our dependency routine is not exactly like apt, small adjustments are sometimes needed.