Play the original first. The voice acting is much better and the pacing and and combat and platforming is still tight. Without the context of the original, Black Mesa won't be as impressive. It's a great effort, but it's pretty clear it's not a professional game.

I'd probably say in lieu. If you like it and it gets you into the HL universe, then going back and playing the original HL is an interesting treat.

Keep in mind Black Mesa isn't quite finished, they are still polishing off the final levels. But there's still a good 10-20 hours or so of gameplay.

"A few days ago"? It only takes like 20 minutes to finish it.

https://youtu.be/VtI5HM7GVGY

I would keep playing through HL1 and then BM personally. There's some changes like to grunt AI (BM is based on Combine AI) that make the original still worth playing.

There's also some features that are broken in the Steam release of HL1 vs the original Sierra release if I remember correctly but that's another bag of worms.

Black Mesa - definitely. Preserves everything in HL1 and adds a lot more. Xen is like 100% different though, Black Mesa's one is waaaaay better but if you want to experience HL1 version...

Play Black Mesa instead. Its the same game but better.

I would say in addition. Black Mesa has rather uneven difficulty which detracts from the experience.