From what I have read, the Krynn games have evil clerics and black robe mages, but those options aren't normally available to the player. Can you hack those classes onto your characters, and if so, are they well behaved?

A similar question can be asked of the Dark Sun games: Can you hack the defiler and templar classes, and are they well behaved?

Far as I've ever read, hex editing Evil aligned classes works just fine in the Krynn games, however -

a) I've never used the hacks myself so I'm very foggy on whether Evil clerics get access to the "reversed healing" spells (Cause Wounds, Poison, Slay Living) or if you just get the same old same old when it comes to spells (mind you, Poison and Slay Living are probably the only really useful ones of the bunch).

b) I'm pretty sure there's no deity-related bonus data included for gods that you aren't meant to be able to pick which means that you wouldn't get any buffs or bonus spells for being a priest of Takhisis :)

c) Black Robe Mages will have an extremely limited selection of spells, because there are no Black Scrolls in the game so all you get is that one spell pick per level up, choose wisely :)

Initial experimentation using Auld Dragon's UGE modules to create a black mage (you can set mage type to '3' rather than '2' (red) or '1' (white), and change alignment to '8' (chaotic evil) ) in Death Knights of Krynn creates a mage who does seem to get bonus spells and can memorize spells normally; spellbook appears to be unchanged as of yet.

It would probably be easier to start in Champions, where the entry fight's not that tough.

Experimenting with a C/F/M in Champions and turning the mage type to '3' (black mage) also works, and the mage does seem to get bonus spells as per the black moon. However, cleric types outside the normal 1-7 (the standard deities) are simply listed as 'cleric', and don't seem to get any special spells. (Training a C/F/M up to 3rd level as a cleric allows him to gain a 3rd level spell, as a neutral cleric, unlike good or evil clerics as listed in the 1st ed Dragonlance Adventures book, so it looks like your evil cleric just follows the neutral rules. Training your mage up to 3rd level, he appears to get the choices of a red mage. So it appears to follow the neutral rules.

What you effectively have by creating an unaligned cleric is a neutral cleric with no special spells or bonuses. A black robe mage acts as a red mage, but can't use red scrolls and is affected by the black moon rather than the red one. So not such a good trade.

Even after training, he does NOT get cause light wounds or cause disease, so it looks like these are forever off limits.

You can indeed hack save files to make characters with evil alignment and black-robe mages as others have said. Technically according to D&D rules an evil cleric could still worship one of the neutral gods (which is my in-game reason for why an evil C\F\M of Shinare is allowed to hang around), but such a cleric doesn't get the reverse healing spells (no big loss in my book as I didn't find those spells very effective in the FR series).