It's been released for a long time now, at least 6 months.
Just hasn't been officially placed on Steam. Just google Black Mesa and you'll find it, drop it into the proper Steam folders and restart Steam and it will be there in your list.
I wouldn't even recommend playing the original Half-Life at this point, anyway. Overrated would be the word for that game, you'll find a lot of old PC players who look back at that game and don't have the greatest memories, just memories of a pretty good game. Half-Life's popularity was brought about by CounterStrike and Team Fortress after it moved from Quake, not Half-Life itself. It just has a vocal minority who praise Half-Life 1 as an amazing game.