Normal maps are predominantly blue in color because the z component maps to Blue and since normals point out of the surface in the z direction we see Blue as the predominant component.

If the above is true then why are normal maps just of one color i.e. blue and they should not be having any other shades(not even shades of blue)

Since by definition tangent space is perpendicular to normal at any point we should have the normal always pointing in the Z (Blue direction) with no X(Red component) and Y(Green component).

Thus the normal map(since it is a "normal map") should have had color of normals which is just the Blue(Z =Blue compoennt = 1, R=0, G=0) and the normal map should have been of only Blue color with no shades in between.

But even then normal maps are not so, and they have gradients of shades in them, why is this so?