Voodoo
Trolls are the most unique race of priests, in that they hold an entirely different belief system compared to the other races of Azeroth. Their priests don't care one bit for Light, Shadow, Elune or anything else, nor do they see themselves in any way connected to the "shadow" magics of the Lich King or the Burning Legion. Instead, their magic revolves around shifting relationships with the spirits that live all over the world. Through ritual and belief, the trolls can influence these spirits for good or evil, whichever way their own heart is inclined.
A troll priest could also be called a Witch Doctor. He could perform all kinds of special rituals for other trolls, including rites to try and see into the future, as well as rites to speak with the dead or cause harm to someone else over a great distance. Evil witch doctors could even perform rites that are forbidden in the new Horde, such as the sacrifice of sentient beings, and ritual cannibalism.
Interfaith dialogue
In the end, whatever the absolute truth is regarding Holy and Shadow energies matters much less than your character's own beliefs about them. If you want to roleplay that your character balances the forces of life for healing and death for destruction because of his faith in the Light, that's fine. Or, you could make your priest into a mystical speaker with the dead, who uses light and shadow magic to probe the mysteries of the soul. There are a whole range of ideas that could make sense for your character.
Alex did a great article about the different religions of Azeroth, which is a great resource for any aspiring priest to find out what his beliefs should be. The important thing is that you should have strong beliefs of some sort, because your divine power comes from them. Even though, due to the necessities of game design, your priest has the same abilities as other priests, your roleplaying can create an entirely different flavor, as if you're another class altogether.
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References:
http://wowgame.weebly.com/1/post/2008/12/blizzard-misspells-arthus-in-wraths-box.html
http://wowgamefun.livejournal.com/2195.html
http://cms.wowgamer.webnode.com/news/new-dalaran-rings-on-the-ptr/
http://wowgamer.oggix.org/2008/12/14/wow-characters/