Are you still seeing this behavior? On my test install running 16.0.6362, I have a solid black background displaying correctly on the login screen. Since this post on the 17th, there have been two releases that address the login page, so it's likely this was fixed with one of those updates... If it's still an issue, do you have the background images field set to Solid Color or Custom Image Folder on Server?
I've marked this thread as Resolved in the meantime. Let me know if I need to update that!