I'm sure there's a logic behind every month's variants. They promote something specific. But I can't find anything related to a film noir or a new movie or anything
The variant covers all have a theme and film noir is just what this month's theme is. I don't think there's any deeper meaning than that. I have no idea why Donald Duck has a Mickey cover. Just like a couple months ago when Uncle Scrooge had a Donald variant cover.
The gag of Donald having a Mickey cover is that Donald has gone missing from his own cover—and that's become the "Case of the Missing Duck," just in time for Mickey to investigate in classic detective style. (-: