Why aren't normal maps just blue? I would think that normal maps should be predominantly blue in color because the Z component of the normal is represented by blue. Normals point out of the surface in the Z direction so we should see blue as the predominant colour since the Z component is dominant.

By definition tangent space is perpendicular to the surface. At any point we should have the normal always pointing in the Z (blue direction) with no X (red direction) or Y (green direction). Thus the normal map (since it is a "normal map") should have the colour of the normals which is just blue (R = x = 0, G = y = 0, B = z = 1) with no shades in between.

But normal maps are not so, and they have gradients of shades in them. Why is this so?