It's not on steam yet, but it's pretty easy to install, just make sure you have SDK 2007 and download the 3GB file from BMS website... there is an exe which will take care of installing the game, after that just start your steam and Black Mesa will be on the list (as a non-steam mod)
it's interesting that on the Half Life 1 page on steam there are news related to the BMS release.
as for the mod, it's great, really impressive, I was expecting less.... I finished it and enjoyed every single moment, it was a bit refreshing to play a game like this,