I assume you have an adblocker running. AG is currently using the background space as advertising space. You can't see this with an adblocker enabled, which makes the background black.
If you have Ad-Block Plus or Ad-Block running, then what will happen is that you will briefly see the Siege Hero ad. When the page is fully loaded, then it will just be black. Or, if you have the Color Change Chrome extension and you have not selected "Use the Site's Default Colors" Option, then it will most likely not appear.
I didn't really mind the adds in the background, i enjoyed looking at them because they were interesting and as long as i knew that Armor Games was earning money and gaining from it, i was happy and i didn't really mind it.