A question came up in a comments thread at "John C. Wright's Journal" about the patron saint of alchemists, with a commenter mentioning that he had somewhere read that it was St. James, but had never been able to verify it. I had always heard that it was St. Roseline de Villeneuve, but I have never been able to verify it from a reputable source. They could both be true, too; there are overlaps.
I do know who the patron saint of sorcerors, magicians, and necromancers is: St. Cyprian of Antioch (not to be confused with the more famous St. Cyprian of Carthage). According to the legends, Cyprian was a practitioner of black magic who gave it all up on conversion to Christianity, which he did when his attempts to use magic to seduce a Christian girl named Justina failed because of her devotion to the cross of Christ; they both became martyrs under Diocletian, and in some calendars have their feast day on September 26. It's one of those things worth noting if only because the legend of St. Cyprian shows up in a lot of places in literature. Pedro Calderón de la Barca, who is perhaps one of the few dramatists to rival Shakespeare in quality, wrote a wonderful play based on the legend, El Mágico prodigioso, which you can read online in a nineteenth-century translation by Denis Florence Mac-Carthy. In any case, the rationale for having a patron saint for practitioners of dark arts is that they certainly need someone to pray for them.
This is a rationale found elsewhere. There is a patron saint of prostitutes, too; his name is St. Nicholas of Myra, and yes, he is the same St. Nicholas whom we associate with Christmas. The reason is that one of his great deeds was to save poor young women from having to prostitute themselves to feed their families. For similar reasons, St. Nicholas is the patron saint of thieves, for which reason you will occasionally find thieves referred to as Knights of St. Nicholas or St. Nicholas's Clerks. St. Dismas is also the patron saint of thieves; Dismas is the name traditionally given to the good thief who died on the cross next to Jesus. The patron saint of murderers, I believe, is St. Julian the Hospitaller.
Of course saint patronage is as much folklore as it is anything else; but it is certainly the case that we need some kinds of symbols to remind us that everyone is worth praying for, in one way or another, whether it's mathematicians or hunters (who share the same patron saint, St. Hubert, whose symbol, a cross-shaped star between antlers, is on the Jägermeister label), or philosophers (whose patron saint is St. Catherine of the Wheel), or sailors (St. Nicholas again), or clowns (St. Julian again), or actors and comedians (St. Genesius of Rome). Whatever one thinks of patron saints and practices associated with them, it's worth considering one of the things the whole practice, and especially its popularity, shows: people often need others to pray for them, and what is more, often need to know that there are others doing so.