Guild Wars 2

More specifically how does it work against bosses with Defiant.

As a Thief frequent Blinds are one of the few utilities I bring to the group. But somehow I haven’t really figured out the exact details of when Blind works and when it doesn’t.

According to the wiki Defiant grants the boss immunity from control effects. With blind being a condition it should effect the boss like it normally would unless the boss also has other immunities (like the dredge which are immune to blind)
For more info http://wiki.guildwars2.com/wiki/Defiant

There is an ability on some bosses that make is so blind only works 10% of the time. That way you can’t blind tank any boss to make it easy mode.