I've been busy with schoolwork, but I did a bit more testing and I think it's pretty clear that blur is interacting with the title bars in a bad way.
As you can see in the pic, the color of the messed up title bar edges (and icons) reflects the color of the background image. Some are greenish, some white, blue, etc.
As well, the visual regression isn't a result of changing any of the highlight code (reverting the highlight stuff doesn't fix it in my patch). It's from moving the title bars over the edges of the windows, so that the blur effect on the edges takes in some of the wallpaper color. The problem is actually there in the code without this patch, it's just not very visible (if at all) because the title bars are centered on the windows.