Atalanta Pendragonne

The Oxford English Dictionary has it as balk. Since it comes from Old English, it only got a 'u' after French had influenced (I am tempted to say 'infected') the spelling system. The 'u' came into the word in the 1600s and 1700s, and today the spelling is very mixed in BrE.  British National Corpus has 21 baulked/13 balked, more recent Corpus of Global Web-Based English has 71 baulked/93 balked in BrE.  Cambridge, Chambers & Collins Dictionary all list balk first as does the Guardian Style Guide.

Most u/non-u variations in BrE and AmE involve an o—as in words like colo(u)r and mo(u)ld. The variation can probably be blamed on Samuel Johnson as much as Noah Webster. In the 18th-century, not all British dictionaries put a u in words like colo(u)r, but Johnson did, and his dictionary became far more famous than the others, so the u form eventually became standard in BrE. I write about the ou/o spellings at this old post (and much more and much better in in The Prodigal Tongue). But even Johnson spelled balk without the u.

My attention to the -o(u)- words in the book meant I overlooked balk/baulk—but I used the word balked in chapter 7:

...please seemed inappropriate in the small request situation, and so Americans balked at it.

The British copyeditor did not bat an eyelash—or a blue pencil—at it.

Given the dictionaries' agreement, we can say that balk is the "standard" spelling of the verb in BrE. Given the corpus numbers, we can say that it is the "normal" spelling. Given the word's history, we can say it is the "original" spelling.

But those corpus numbers aren't so distinct, and given the conversation on Bluesky, it seems that some BrE writers really want to spell it with a u (and to believe that that is the "standard/normal/original" way to spell it). This may well be another instance of British spelling changing in recent decades in order to fight against perceptions but not realities of a British/American spelling difference.*

Confusion about its spelling is understandable, though, since there is a noun that is more usually spelled/spelt baulk. It's part of a billiard-type game table, and the term is used in several terms in several such games.

Illustration from International 8 Ball Referee

I searched for several of these terms as either balk or baulk in Corpus of Global Web-Based English and got only the u spelling—with none of them in AmE:

(Britannica.com has it as balk. They're owned by the same company as Merriam-Webster. Is the ghost of Noah W. removing U's in the encyclop[a]edia?)

So, in general, if it's a [billiards] noun, it's baulk and if it's a verb it's balk. They both come from the same Old English word, with a Germanic ancestor.

The one other -aulk word in current English is caulk (used much more in AmE than BrE, which tends to say seal/sealant instead)—but that came into English with the u, as it came from French cauquer in the 1500s.

The post-Norman urge to stick a U in balk also affected talk and stalk in the 1600s, but not, apparently walk. For me, the mystery is: why has the urge to stick a U in persisted for baulk and not the others?

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*I say 'another' instance of spelling change due to perceptions of 'Americanism' because I discuss the main instance of that phenomenon in detail in The Prodigal Tongue: