As mentioned in the "absolutely unheard of" post above, I was in the process of getting Dark Magic for my party when I got a Titanic Ttrident of the Dragon from an Enchant Item spell. In due course, I donated 10x to a Baa Temple (reputation down from Saintly to Average), cleared out the Superior Temple of Baa and took advantage of the Baa Heads (reputation dropped from Average to Bad), sold my collection of bones to the collector in Rockham (reputation plummeted from Bad to Notorious), and went to visit Su Lang Manchu to get my party Dark Magic Master.
This was immediately followed by Portalling back to New Sorpigal, donating a few times to the Temple there (reputation shot back up from Notorious to Respectable), and going to make my long-postponed visit to the Lair of the Wolf to do all the quests in there, converting the werewolves to "waswolves" in the process. I then popped over to Silver Cove to deliver suitable admonishment to Cedric Druthers and his coterie of corrupt druids (and their friends) in the Monolith. After that, I checked on the health of my party (Shift-Z) and greatly to my surprise, my reputation was back up at Saintly.
I do not remember ever playing an MM6 game in which my party has achieved a Saintly rep after acquiring mastery in Dark Magic.