[O] Implement a new emphasis style to hide text

[Stefan Janik]

Hello, 
for some purposes I need to hide some words. (It would help to memorize these
words). In the enriched mode I can do this just by marking the words and
choosing M-x facemenu-set-background and then typing "black" for the "Background
color". 

In the org-mode the enriched mode doesn't work. 

A perfect solution for me would be a new emphasis style: 
Maybe something like $a hidden text.$ --> $JUSTABLACKBOX$
(like the _underline style_ ). 
How can I do that? (Or any other suggestions?)
 
Thanks.

Best regard, Stephan