Looking for a book

[Foustian]

There's a book I read several years ago, which I believe was the first in a series. It was really good, and I've always wanted to see if there were more, but I could never remember the name or author.

The story was about a young boy who, unknown to him, can control elementals (which is a really rare and powerful ability). He befriends a group of magicians, one of whom is blind in the normal sense, but can see with his mind. I remember one part where he was putting on a carnival show of blindfolding himself and throwing daggers at his assistant. The assistant then gets really nervous when the guy manages to dodge a knife that's thrown at his back, and wants to know how he knew it was coming. There's a woman magician too, who I think can make illusions. There's a part where the group disguises themselves as bushes, but are almost caught because the horses of whoever is chasing them can smell them.

There's also this old Gandalf-type guy, who made special contact-type things for the blind guy that look like real eyes (since his are pure white, and he would be persecuted if people knew about his ability). At the end, the group is fighting some evil guy, and the Gandalf dude is there, and they're getting their asses kicked. Gandalf unlocks the kid's power and the kid summons incredibly powerful elementals to defeat whoever they're fighting.

After that Gandalf is gone, and the group decide to look for him. The last part I remember is the kid summoning a small fountain of water in the desert to practice the minimal control he now has over his power due to what Gandalf did to him.

I know that's pretty vague, but if anyone has any idea of what the book might be called, or who wrote it, it'd be greatly appreciated.

[Dusky]

Blade of the Poisoner, by Douglas Hill.