This is a short one:
Can a rogue under the effects of blur spell, but in plain sight, make a hide check?
Hide says under concealment he can... but just doesn't sound right, does it?
No, he can't. There are two separate conditions that must be met:
Blur provides this. However, the second condition is as follows:
And Blur does not help with that. You need a separate ability for Hiding in Plain Sight, such as the Ranger's or Shadowdancer's aptly named class features.
It depends: Is anyone watching him?
However, you need more than concealment to hide:
(Source, emphasis added)
So, if your rogue is alone in an open field, the Blur spell gives him just enough camouflage that he can attempt a hide check.
On the other hand, if someone is watching him, Blur won't allow him to suddenly disappear.
Somewhere between the two, the rogue can pull the old "Hey, look over there" trick by making a bluff check, and then attempt to hide using Blur.
In general, there's probably enough clutter in the environment to make this kind of thing plausible. The rogue is hard to see, so if there's something to help conceal him, he uses it to hide.
Of course, if you're actually fighting in a featureless plain, you may want to add a situational modifier...