Fwd: Why does big black box appear over by GRUB2 theme when booting?

[Ed Hartnett]

I made a nice GRUB2 theme, which gives me a screen like this:

http://s33.postimg.org/qe6gm7e0f/screen_small.png

When I make my selection and start to boot, for about 10 seconds a big
black box appears over the screen. Then the whole screen goes away and the
boot proceeds.

It looks like this:

http://s33.postimg.org/ehb5qqkkv/screen_blocked_small.png
​
Why is the black box appearing? Is there any way for me to stop it showing?

The files for the alien theme can be found on GitHub here:
https://github.com/edhartnett/alien_theme

Thanks,
Ed