Certain websites are "blacking out" because of the SOPA/PIPA (Stop Online Piracy) laws that could possibly censor the internet. It was mainly proposed because of the illegal downloading stuff on websites like BitTorrent and Limewire. However people believe that the bill will completely censor and allow government control of the free-world known as the internet. Obviously people don't like this. Instead of "blacking out" Google just put a dark black box over the logo.
Wikipedia blacked out the other day
Reddit is planning on blacking out as well (or so I've heard)
I think Google should blackout too. I mean, I feel like it would send more of a message if people couldn't use their precious Google. (I know I use it for everything). However it still brings a ton of awareness to what is going on because a ton of people still don't know about it. At least since Google isn't blackout people will question what the logo thing is all about and maybe "google' their curiosities and find out all about SOPA and why it is such a big deal to some people.