The Dropbox applet / panel icon is black and not transparent as compared every other applet icon. I noticed this only happens when running proprietary NVIDA drivers.
I see exactly this (also like this on 15.04) with proprietary Nvidia drivers. I never noticed how it looked before installing them.
I spent quite a lot of time trying to track down the reason, but failed. I never saw this with any other desktop (XFCE, unity). Not the end of the world, but a bug that likely affects many people.
You could file a bug report using your screenshot. Button top right “issue tracker”.
D.