It appears this spell has been nerfed unfortunately, in the previous iteration the creatures only had to be present within 30 ft during the casting. Now it states that they must remains within 30 ft of the caster for the duration 

Quote from SGTJetMaster >>

It appears this spell has been nerfed unfortunately, in the previous iteration the creatures only had to be present within 30 ft during the casting. Now it states that they must remains within 30 ft of the caster for the duration 

That's true, but it also means that allies that move into the affected area after the initial casting can be affected. You don't necessarily need to worry about excluding someone because they happened to be out of range when you cast it. It's less of a direct "nerf" and more a lateral change.

Does the blinded bit apply only when the target of the attack is a chosen creature, or does it apply across the board?

for example, if i cast this, and an undead ally of mine uses a melee attack on an enemy unit within the aura, would it risk the blinded condition?

Quote from rem3die >>

Does the blinded bit apply only when the target of the attack is a chosen creature, or does it apply across the board?

for example, if i cast this, and an undead ally of mine uses a melee attack on an enemy unit within the aura, would it risk the blinded condition?

It says "when a Fiend or an Undead hits an affected creature with a melee attack roll". "Affected creature" in this context means one of the creatures you've chosen for the spell to affect.

So, in your example, your undead ally's attack against an enemy does not risk blindness — unless for some reason you chose to protect the enemy with this spell.