Yes, a Human Freemage can learn both Dark and Light magic skills. S/he can become a Grandmaster in Dark, but only a Master in Light. Dark Magic is the only skill set that allows certain secret doors to be visible. However, there are beaucoup potions that allow anyone to see any secret doors for about 30 or 40 turns. Furthermore, there is a certain animal who is imprisoned by a band of tough brigands. Freeing this animal will earn you its gratitude forever. It can even become a member of your party. Oh, did I mention that it also has the ability to see secret doors?
Dark Magic isn't nearly so powerful in MM10 as it is in MM7. Some of the Elemental spells are as deadly, if not more so.
If you're really keen to try out Dark Magic, you must have a Human in your party. Other than this, Humans aren't really very valuable in this game. Orcs and Dwarves are the most powerful with a weapon; Elves, Dwarves, and Orcs all can have healing powers--with their prowess in the order listed.