They don't take all of it, so if they're doing what they're supposed to, nearly all the crabs will be returned to the ocean within a few days and eventually be fine again. Some do die, though, and even if they don't, they're worse for wear after the process, and some companies have been accused of taking all the blood and then selling the dead crabs as fishing bait. There's an artificial alternative available, but regulators aren't all convinced it's as effective, so it isn't used universally yet.

https://en.wikipedia.org/wiki/Horseshoe_crab#Use_in_medicine