The Fey Ancestry racial trait shared by elves (Player’s Handbook, p. 23) and half-elves (p. 39) states “magic can’t put you to sleep.” Warforged’s Constructed Resilience trait is similar: “You don’t need to sleep, and magic can’t put you to sleep” (Eberron: Rising from the Last War, p. 36). Kalashtar have a racial trait called Severed from Dreams, which reads (p. 31):
Kalashtar sleep, but they don’t connect to the plane of dreams as other creatures do. Instead, their minds draw from the memories of their otherworldly spirit while they sleep. As such, you are immune to spells and other magical effects that require you to dream, like dream, but not to spells and other magical effects that put you to sleep, like sleep.
The spell dream of the blue veil begins with the following paragraph (Tasha’s Cauldron of Everything, p. 106; emphasis mine):
You and up to eight willing creatures within range fall unconscious for the spell's duration and experience visions of another world on the Material Plane, such as Oerth, Toril, Krynn, or Eberron. If the spell reaches its full duration, the visions conclude with each of you encountering and pulling back a mysterious blue curtain. The spell then ends with you mentally and physically transported to the world that was in the visions.
A sidebar next to the spell reads (p. 106, emphasis mine):
Transit between [different worlds on the Material Plane] is rare but not impossible and can be accomplished in various ways. […One] method is the Dream of Other Worlds; travelers fall into a deep slumber and dream themselves into a new realm. The spell dream of the blue veil employs this method of transit.
My question is, can elves, half-elves, warforged, and/or kalashtar be affected by dream of the blue veil?
My current interpretation of RAW is that the answer is “no” for all four creatures. The sidebar outright states that dream of the blue veil induces “slumber” (thus excluding elves, half-elves, and warforged from being affected) and causes its targets to dream (excluding kalashtar). However, there are a few reasons I might be wrong.
Nowhere in the spell’s actual text do the words “sleep,” “slumber,” or “dream” appear. It refers only to creatures becoming “unconscious” and experiencing “visions.” Contrast this with sleep, which “sends creatures into a magical slumber” and refers to its affected targets as “sleepers” (PHB, p. 276). Contrast also with dream, which “shapes a creature’s dreams,” doesn’t affect a target until it’s “asleep,” and explicitly states that “Creatures that don’t sleep, such as elves, can’t be contacted by this spell” (p. 236).
Commenters in this Reddit thread largely agree that the lack of sleep- and dream-related language in the spell’s text means RAW is that kalashtar can be affected. One commenter points out the similarity to the spell catnap, which can affect elves.
Maybe the Tasha’s Cauldron sidebar, like the title of the spell, is simply some extra lore that doesn’t affect gameplay. However, I see no reason to exclude sidebars from RAW. Many important rules, such as the ones for hiding (PHB, p. 177) and some rules governing wizards’ spellbooks (p. 114), appear only in sidebars.
Another Reddit commenter points to “Spell Spotlight: Dream of the Blue Veil,” a D&D Beyond article which contains the following sentence:
A kalashtar wizard who uses the dream [created by dream of the blue veil] to flee the agents of the Dreaming Dark by escaping to Krynn may buy themself enough time to gather new allies, new resources, and return to Eberron with a bold new plan to defeat their enemies.
This obviously implies that kalashtar can be affected by the spell. I imagine that this could be an oversight, and I don’t especially see why this article should be considered RAW. But again, I could be wrong.
Best Answer
This kind of thing is very much in the realm of how the DM's world works.
From a worldview standpoint, I would say it works on all of them. The spell doesn't say "sleep", and all of those races can become unconscious. I would not treat the sidebar as RAW.
The Kalashtar question is a bit trickier, but since these visions come from the world to be traveled to (which is a real place, in the game), and not from the "plane of dreams", I would say it works for them as well.
On a more practical gaming level, the spell is a plot device to get the party to the place the story needs them to be at. Ruling that some of the party can't get there is definitely not a way to have fun and improve the game.
(Unless you make that part of the story. I could envision a campaign in which the spell is the last event of a session, and subsequently the DM splits the game into two sessions -- one with the players whose characters went to the new world, and realize some didn't get there, and they have to figure out (a) whatever the start of the adventure there is, and (b) how to get the rest of the party there [e.g. they figure out the key note of the new world, make a tuning fork to match it, someone Plane Shifts back to the old world, and uses the new tuning fork to bring everyone to the new world] -- and the other session with the players of the characters that didn't get to go, and they have to figure out how to get there [e.g. go take out a mindflayer colony and steal their Nautiloid, and sail it there]. With both groups proceeding independently, that could be a fun adventure idea in itself.)