It is not a Source port, it is the original Blue Shift.
The only way I know you can get it is if you had the collectors edition of Half Life or you have the original Blue Shift CD. Also if you bought one of the Half-Life packages which I think Silver and Gold would be eligable for you to download Blue Shift.
You also need the original Half Life to play Blue Shift.
The ability to install the content required to play Blue Shift. They said they'd be releasing the high-definition pack too, I think sometime in the next week (but note, those are Valve's words, and TF2 was supposed to be out in, what, 2001?).
I have the old "Game of the Year" edition, and I got Blue Shift, so that must be included in those packages as well.
Yeah, I think you can get it with pretty much any version of Half Life. I have a very old GotY edition that came with Opposing Force, and it's showing up on my Steam list to install. I might give it a try, can't remember if I ever finished it or not.