How exactly does this feat work and whitch spells am I able to sunder ?
From feat description:
You can attempt to sunder an ongoing divine spell effect or divine spell-like ability by succeeding at a combat maneuver check.
It's easy to understand for spells like Ice Armor where I can see effect of the spell but what about spells like Calm Emotions ? I just saw enemy priest casted something on my barbarian friend and he stopped ragging. Can I use sunder blessing here ? What would be target of my sunder ? Aura around my barbarian friend ?
What about buff spells ? What about aoe spells like Zone of Truth without any visual effect.
The rest of the description you quoted answers your questions:
For any effect other than one on a creature, you must succeed at a combat maneuver check against a CMD of 15 + the effect’s caster level. To sunder an effect on a creature, you must succeed at a normal sunder combat maneuver check against the creature’s CMD + 5, ignoring any miss chance caused by a spell or spell-like ability. On a success, you suppress the effect for 1 round. If you exceed the CMD by 5 or more, you suppress the effect for 2 rounds. If you exceed the CMD by 10 or more, the effect is dispelled instead. If you fail to sunder a spell, you cannot attempt to sunder that effect again for 1 minute.
Yes, this means that in the case of calm emotions or other negative effects on creatures, you're targeting your barbarian friend's CMD + 5. The feat is (with apologies to the designer) not well-designed, as it doesn't really account for sundering bad effects on creatures.
Effect visibility doesn't appear to matter--you can sunder it anyway.