Cloud of Darkness traditionally doesn't make areas dark, it actually creates a globe of black passable "darkness" that is impossible to see through. It also can be dispelled via clerics and other holy powers in older editions I believe . I would vote that Cloud of Darkness wins.

Let's take a look at the two powers in question

from Firecrafter...

" Also, you can emit dim light out to 5 squares. As a minor action, you can suppress or resume this light. "

Cloud of Darkness

A shroud of blackness descends around you, hiding you from sight.

Encounter  
Minor Action      Close burst 1

Effect: The burst creates a cloud of darkness that remains in place until the end of your next turn. The cloud blocks line of sight, squares within it are totally obscured, and creatures entirely within it are blinded until they exit. You are immune to these effects.

On page 29 of the Rules Compendium it states that Specific Beats General, I would say the specific nature of the cloud of darkness beats the general feature of the Firecrafter. But it's your game and at the end of the day it is up to you to decide. As you know I personally would favor Cloud of Darkness. You could use this to describe how the monks light ahead of him is suddenly engulfed in a globe of pitch black darkness or something to that tune.