Although it might present itself as a cosmetic update, Black Mesa actually changes a fair amount of stuff. You get weapons in a much different order, the weapons themselves have undergone balancing tweaks, dialogue has been added or rewritten, many level segments have undergone significant redesign... the thing I found most noticeable, though, was that a lot of the sandboxiness of the original game was gone. In Halflife, puzzles had multiple solutions and there were a slew of quirky little hidden ways of doing things. Black Mesa took most of that out.
I'd play Halflife first, then come back and play Black Mesa some time down the road.
EDIT: oh, guess you already made your decision :P
Veneteaou: If you bought the whole collection, you should really play the original first. That way, you don't ruin yourself by having to go backwards to play Opposing Force (IMO the best title in the series).
I actually really liked Blue Shift. It was really short and lacking in content compared with Halflife and Opposing Force... but I remember it as feeling a lot more focused, having more enjoyable encounters, and having the best incarnation of Xen. Plus the story seemed a lot more cohesive, rather than just being a wild run from location to location with no real recurring characters or sense of purpose.
I have really fond memories of Halflife and its expansions. I remember it being so easy to turn off all the lights and get immersed in them way back in...umm...2007, I guess?
Yeah, I've always been a little behind the times :P
movieman523: Xen was by far the worst part of HL1 anyway; they should leave it out permanently :).
The platforming certainly sucked, but I thought Xen itself was rather awesome.
