###Radiant damage, despite its name, is not damage caused by light.###

Radiant damage simply happens to *look like* light. Look at the examples of radiant damage: a cleric's flame strike spell, or an angel's smiting weapon. Radiant damage is caused by *holy power*; it looks like light, yes, but that's a side effect.

Consider the 3.5e version of [Flame Strike][1], which deals half fire damage and half "divine power" damage (since the radiant damage type didn't exist in that edition). Similarly, in 4e, where radiant damage was first introduced, it was dealt almost entirely by divine classes; almost all undead were vulnerable to it, despite most being able to walk freely in sunlight.

In general, spells do what they say they do. Darkness spells don't say that they prevent radiant damage in any way, so they don't. Silence spells *do* explicitly make their subjects immune to thunder damage, so we can see that the designers haven't simply forgotten that these area spells might have such an effect.


  [1]: http://www.d20srd.org/srd/spells/flameStrike.htm