Am I reading correctly that when an ally targets you for a heal spell for exemple while you are under the effect of a blur spell, they have to make the flat check as well to see if they can target you? And what if the spell comes from an area effect like bless, do you still need to make the flat check? I'm trying to figure out when you need to make the flat check. As I understand, it's whenever something targets you whether it's from a strike, an ability or any spell? Is it only for direct target?
Yes, they need to target you. It's like how if you were invisible they'd have troubles too. I remember a player's cohort who got paralyzed while invisible, yet the enemy has See Invisibility. So the party could do little to aid her and she soon died.
Area spells aren't effected by this, again much like invisible won't spare you damage from a Fireball. Unless I guess the caster has to make targeting decisions within an area? (if there are any such spells/effects)