I'm finding the wording... vague.
Have a neutral cleric channeling negative energy. Decided, make it interesting with alternate channel: darkness.
Harm—The illumination level in the area drops by 1 step, as darkness, for 1 minute. At cleric level 10, the light level drops by 2 steps. At cleric level 15, it drops by 3 steps, and areas of dim light or darkness become supernaturally dark (even creatures with darkvision cannot see within it).
So, normal variant rules: Always half damage, will save to negate the secondary effect.
I would think this means, half damage, fail the save the channeled area appears dark. Make save, isn't dark. Similar to the darkness 1st level domain power, touch of darkness.
But the 'as darkness' part concerns me. Darkness just makes darkness, in an area, no saves for anyone. Is the channel area just getting a darkness spell cast on it? In which case, even with selective channeling I'm blinding my allies? And come level 15, even with dark vision, I'll blind myself?
Fire's alternate channeling doesn't effectively throw a fireball at his feet... so why would darkness?
Well...I think yoo're right. I believe it just creates darkness. And at level 15 it will create supernatural darkness which you can't see through. There are ways to mitigate that at level 15, though having them available all the time might not be easy.
I don't see any saving throw mitigating that effect. Enjoy! Or perhaps choose another option if you don't think your group will enjoy it.
Interesting to note that it has to start dim or dark to become supernaturally dark (at 15th level), even when normal light only takes two steps to bring to darkness.
Yeah. That's the conclusion I'm coming to. It's of merit to note that the Darkness Domain grants blind fight and 'eyes in the darkness' at level 8, (SU: see in darkness and magical darkness for 1/2 cleric level in rnds).
As a cleric of Greteous, a little friendly fire with vision isn't out of place (I do have selective channeling so they're at least not getting neg'd).
Bonus question, since it's as Darkness, take a look at this:
Nightmare Weaver (Combat)
You can use your ability to create magical darkness to terrorize enemies.
Prerequisites: Improved Unarmed Strike, Nightmare Fist, Intimidate 2 ranks, ability to cast darkness.
Benefit: By spending a full-round action to cast darkness, you can also make Intimidate checks to demoralize all foes in the spell's initial area.
Special: This feat counts as Dazzling Display for purposes of qualifying for Deadly Stroke and Shatter Defenses.
Think you could channel as a full round action and scare the crap out of everything?