Targets in the affected area are definitely blinded, regardless of the light level. The dust is a conjuration effect, and particles can still blind you with no light. So the spell still has a use against enemies with Darkvision.
As for the effect on invisible targets, the spells says that it covers the area, "visibly outlining invisible things". With no other relevant text, I would make a default assumption that this "visibly" refers to something that is normally visible to the caster. It would not make the target visible if the caster were blinded. It would not make the target visible if he was behind cover. And it will not make the target visible in complete darkness.
It's like drawing a line around the target. You can draw a black line on a black piece of paper, and the line is still there even if a normal person can't see it. The outlining would still be useful for a caster (or friend) with Darkvision, though.