It'd certainly be interesting if there was textual evidence that most/all Rohirrim were blonde and most/all Gondorians were dark. This would hint at very little if any intermingling between the two nations and as such would make them insular to an incredible degree. As Lommy says, no real nation has an 'exclusive' colouring (though some colourings may be more common in some countries/regions - you see a LOT of ginger Scots!) - this is due to intermingling which goes back for millennia.
Maybe of course this underlines one of the points that Tolkien was trying to make, that the race of Men was not even nearly as united as it ought to have been. So their 'sins' in attitude emerged in their appearance?
Anyway, I want to know where the redheads were?
