[Merlyn] said you could not use magic in Great Arts, just as it would be unfair to make a great statue by magic. You have to cut it out with a chisel, you see. (S.4.5)
It seems that there's a sort of Code of Magic, similar to the Code of Chivalry. Magic's okay for some things, but you can't be a big fat cheater and use it for everything. Since hawking is considered one of the Great Arts, Merlyn refuses to capture Cully by using his Jedi mind tricks.
"Now the Beast Glatisant, or, as we say in English, the Questing Beast—you may call it either [...] This Beast has the head of a serpent, ah and the body of a libbard, the haunches of a lion, and he is footed like a hart. Wherever this beast goes he makes a noise in his belly as it had been the noise of thirty couple of hounds questing." (S.2.49)
Sounds like something Dr. Victor Frankenstein would dream up as a pet for his monster. The Questing Beast is a magical creature that King Pellinore pursues, to show the ridiculousness and frivolity of some knightly quests.
"Some people say they are the Oldest Ones of All, who lived in England before the Romans came here—before us Saxons, before the Old Ones themselves—and that they have been driven underground. Some say they look like humans, like dwarfs, and others that they look ordinary, and others that they don't look like anything at all, but put on various shapes as the fancy takes them." (S.10.112)
You can't have a good old-fashioned story about the medieval British Isles without adding faeries to the mix! Robin Hood is telling Wart and Kay about Morgan le Fay, whom we find out in the following book is, indeed, one of the Old Ones. She and her sister, Morgause, have inherited an ability to use magic. It's almost a letdown when we see how Morgan uses her vaunted abilities to turn things into… food. Boring.
She was not a serious witch like her sister Morgan le Fay—for her head was too empty to take any great art seriously, even if it were the Black one. She was doing it because the little magics ran in her blood—as they did with all the women of her race. (Q.1.52).
White distinguishes between greater and lesser varieties of magic, and also notes that certain races are more attuned to magical frequencies than are others. So, the women of the North—the Old Ones—like Morgause here, are more adept at magic than others.
[T]here came a magic barge from over the water, a barge draped with white samite, mystic, wonderful, and it made a music of its own accord as its keel passed through the waves. Inside it there were three knights and a seasick brachet. Anything less suitable than these to the tradition of the Gaelic world, it would have been impossible to imagine. (Q.5.60)
These novels are just lousy with magical ships, barges, canoes—you name it. If it sails, there's bound to be a magical version of it somewhere in these pages. But seriously: the first magical barge we see shows how magic and adventure can be linked. Good knights don't turn down an adventure, even if they have a hot lady they're trying to chat up (like King Pellinore). So, Pellinore, Sir Palomides, and Sir Grummore—along with Pellinore's trusty brachet—all hop into the barge and set sail for... who knows. They end up in Orkney.
"Now," says she, "I could make the ship be struck against the coast." "You could not do that," says the father. "Well, look at me now," says the little girl, and jumped into the well. The ship was dashed against the coast and broken into a thousand pieces. "Who has taught you to do these things?" asked the father. "My mother. And when you do be at working she teaches me to do things with the Tub at home." (Q.7.48).
One of the patterns in White's novels is that magic is passed down from mothers to daughters, or among sisters. Like with the Beautiful Cornwall Girls. So, in a way magic is largely a feminine realm, since Merlyn seems to be the only male practitioner.
"I have a Master called Bleise who lives in North Humberland, and perhaps he will be able to tell me what it is I am trying to remember." (Q.10.21).
In Malory, Bleise (Blaise) is Merlyn's Master, to whom he tells the history of Arthur so it can be written down. In the novels, we don't hear too much about this guy. This is pretty much it. Is this who has taught Merlyn magic? That might be hinted at here in his title "Master" (which in the medieval period could mean either "Ruler" or "Teacher").
The strip was a less cruel piece of magic than the black cat had been, but more gruesome. It was called the Spancel—after the rope with which domestic animals were hobbled—and there were several of them in the secret coffers of the Old Ones. They were a piseog rather than a great magic. Morgause had got it from the body of a soldier which had been brought home by her husband, for burial in the Out Isles. (Q.13.39).
First of all: Eww! We're edging into Buffalo Bill territory here. Morgause uses this strip of human skin to bind Arthur to her. For a "piseog" (little magic), this act certainly has major repercussions.
Facts of which Guenever was subconscious, in this sense, included the whole of the Arthur-Lancelot situation, most of the future tragedy at court, and the grievous fact of her own childlessness—which was never to be remedied. (K.16.5)
Otherwise normal, Guenever seems to possess a sort of low-grade clairvoyance, which works on an unconscious level. She knows things are going to happen, but she doesn't know that she knows. You know?
Their faces were fanatical, and they babbled of dreams. Ships which moved of their own power, silver tables on which strange Masses had been said, spears which flew through the air, visions of bulls and of thorn trees, demons in old tombs, kings and hermits who had been living for four hundred years—these figured in the rumours which filled the palace. (K.28.2)
The Grail Quest definitely falls into the realm of the supernatural. And by supernatural in this case, we mean God. Literally supernatural: "above nature." It's heavily implied that he's in control of this particular quest, since it's all about teaching the knights to be spiritual instead of worldly.