Check the list of confirmed participants yourself. Individual tumblrs have, however, elected to black out their blogs for a day. You can do this too.
But there has been no mention of a site-wide black out. You’d think Tumblr would let us know with a big banner at the top of our page or something.
“And some of the biggest names online, including Google, Facebook, Twitter and Tumblr, have vocally opposed the proposed legislation, though they have not said they are joining the online blackout.” (source)
Please spread the word.