How did 'knight' semantically specialise to mean acquirer of a target company?

There could be a case made for it, but I don't think it comes from or has any direct relation to chess. In the context of business mergers and acquisitions (M&A), there are four types of "knights." These colored knights are used to represent the various roles of participants in an actual or potential company takeover.

A black knight is a company that makes an unwelcome takeover offer for a target company. This hostile takeover may proceed via a tender offer, a proxy fight, or by purchasing shares on the open market.[1]

A white knight refers to a type of defense against a black knight bidder. While the target company still loses its independence, the white knight investor is more favorable to shareholders and management.

A white knight investor describes a friendly company arriving at the messy scene of a hostile takeover and acquiring the target company. The white knight may slay the hostile bidder with a higher bid or a more favourable deal for the target board's management.[2]

In addition to the white knight and black knight, there may be a third corporate entity, called a gray knight. This unsolicited bidder enters the scene in order to take advantage of problems between the first bidder (the black knight) and the target company, possibly even trying to takeover the black knight (predator) company.[3] Although friendlier than a black knight, the gray knight still seeks to serve its own interests, and is not as desirable as a white knight.

A yellow knight is a company that was planning a hostile takeover attempt, but backs down into a merger with the target company, following an aggressive acquisition attempt.[4] (If present in the transaction, the yellow knight is usually the same company that was initially the black knight.)

Clearly the word "knight" has kept its original connotations, as it symbolizes power, valiance, and sometimes benevolence. I think that the colors of the knights are what sparked the application of the general word "knight" to business lingo.

From dictionary.com, "The notion of a white knight dates back to at least the 12th century, when French poet Chrétien de Troyes referred to the legendary Lancelot as the Blanc Chevalier (“White Knight” in French) in his Arthurian writings."

White symbolizes purity and health in most cultures - this is why you will commonly see rescuers in movies on white horses. A white knight acquisition is when a company will "rescue" a financially struggling entity through acquisition.

Yellow symbolizes many good things, but also cowardice. A yellow knight takeover is when a company is initially aggressive/hostile in their takeover attempt, but mellows out and proposes a merger eventually.

Gray symbolizes shade, dullness, and concealment. A gray knight comes out of the shadows as a second corporate bidder in an acquisition.

Finally, Black symbolizes ill intent and darkness when it comes to acquisitions, which is why black knight acquisitions are predatory in nature.

Overall, the term "knight" keeps its same meaning and connotations, but when put together with colors, it changes the "intentions" of the knight. It doesn't have to do with chess or a semantic change of the word itself.