I’m thinking of trying out a Hexblade Warlock in Adventurers League, which limits players to PHB+1 for character-building.
The problem I have with this is that the Hexblade is from Xanathar's Guide to Everything, but I really want the booming blade cantrip as well from the Sword Coast Adventurer's Guide. Is this legal in Adventurers League play?
As a warlock (and not of the Tome), the only way you learn spells is either by selection at creation or upon advancing a level.
That you can only use materials from PHB+1 for creation-choices is well detailed in the first page of the Adventurers League Player's Guide. So you cannot, at creation, choose both Hexblade and booming blade.
Now flip to the last page of the ALPG, "Character Advancement."
Feats and spells gained by advancement are subject to PHB+1.
So you can't gain booming blade through advancement, either.
Note that spells copied into a spellbook do not fall victim to the +1 restriction, as they're neither gained at creation nor through advancement. But note further, that's a spellbook, not a grimiore
The problem I have with this is that the Hexblade is from Xanathar's Guide to Everything, but I really want the booming blade cantrip as well from the Sword Coast Adventurer's Guide. Is this legal in Adventurers League play?
"Plus one" means only one "plus one" per character.
From the Adventurers League Player's Guide linked on the AL Resources page:
In addition to the Player’s Handbook however, you may also choose to use one other resource from those listed below to build your character.
