FYI, the register form doesn't display for adblock plus users -- forms usually do, so this was quite confusing. It looked like you weren't accepting new users.
Re: adding a black box.
It looks like just not using "mc_embed_signup" and "mc-embedded-subscribe-form" as the form's id and class would solve the problem. Elements with those classes and ids are display:none'd by ABP.
(You can verify this yourself by enabling adblock plus, then inspecting the element)