I bet it has to do with stress, and blue has been shown (with green) to be good for that. And of course, there's a pretty girl in the picture lol
Interestingly, there's like a whole palette that game/movie/book covers will use to be more eye-catching. You've probably seen it too, but not realized it at the time. It's that blue/yellow mix they do that really stands out. Some of Brandon Sanderson's books show it.
That's a good point. I thought that blue was the color that disrupted sleep though? Which is why it is often filtered out by those different "easy sleep" apps. Interesting that it can help with stress. And sure, the pretty girl likely helps :)