How does Cancel Magic work?
 

I was in a group tonight where the tank was being blinded, I wanted to use Cancel Magic but wasn't sure if it would remove buffs and end up being a net waste. How does Cancel Magic determine which spells to remove? And can it cure Blind?

In my experience, and I'll defer to anyone who has gazed at the inner workings of this, it takes buffs and magic-based debuffs off more or less right to left (it can jump a bit). It will not affect things that need to be cured, such as diseases, poison, and unfortunately, blindness. You need a cleric or a shaman for curing blindness.

Every dispell has a random dice roll on it's "strength". 1-4 or 1-8, depending on level of spell.

Every buff and Debuff has a random dice roll on its strength. 1-8

Basically the Dispell will go top down removing whatever number it rolled on its strength until it completely removed a buff. Then it will stop. This means if you have 15 buffs all with 8 strength, and you cast a cancel magic, which rolls a 1 on its strength roll. Cancel would go through every buff and remove 1 counter of strength leaving you with all 15 buffs but they would now all have 7 strength.

However if you had a buff with 1 strength in your top slot cancel magic would remove that and stop removing any counters of strength from the rest of your buffs.

I believe they made a change to make it so that most Clickies are a low strength roll which would allow them to more easily be dispelled. Other than that I'm pretty sure it's all random.