I swear that this was already on steam back when I downloaded it, but now I only see a green light page for it. I want to play hl1 very bad, I have played 2 and both chapters. Will I miss much if I don't play blue shift?

Black Mesa is just a remake of the first one, and Blue Shift is just an expansion pack for the first one. So no, you won't miss anything. And you can always buy Blue Shift afterward.

I couldn't play Black Mesa because it would give me such a headache and make me nauseous. That's a shame, because I'd really like to know why so many loves the HL series so much.

I think you can disable head bobbing in the options menu.

Bear in mind that Black Mesa only covers the first half of the game at the moment, the second part will be released later.

I just beat Opposing Force and I'm about to start Blue Shift.

Fun times.

Storywise, not at all. All that happens in Blue-Shift is that you play as one of the security guards who has to escape the facility during the first 24 hours of the incident. By the end of the story, you've only done just that. Aside from a few insignificant bits of lore that the game adds to the universe, there's nothing else to be gained from it.

Half-Life 1 broke new ground for its time mostly with its realistic and highly interactive world design, but those elements have been copied and recopied into so many shooters since then that it's hard to look back and notice anything really extraordinary about it. Black Mesa is a really nicely done mod for giving the original game such a substantial face lift, but it's still hard to appreciate it unless you were there to play the original when it was still new and cool.

In short: Half-Life, like a lot of shooters, simply hasn't aged very well.

"Half" is an understatement. The entirety of Xen takes place during the final three chapters of Half-Life's 18 chapter story.

Black Mesa covers most of HL1, but is only half-finished. They said they are drastically expanding the Xen bits from what they were in HL1 and will make up the second half of Black Mesa.