This suddenly appeared at the top of my screen. When I hover just over the 'buy' text I get an option to add something to my wishlist, implying that there's some sort of item behind there? I don't know. Is this a bug or is meant to be there?

yeah, that would be the 'hustler flash sale' banner, which has suddenly appeared outside of the cash shop. mine wasn't black, i could see the information, but it was kind of messed up and not looking how it should, and also it was covering the navigation tabs.

Oh, okay. It's gone now anyway...

The color is based on your site layout. But yes I got that too, and I also got a banner that also appeared how it's apparently supposed to, when the item changed just a little bit ago.