It is shown that the shear-induced distortion of the static structure factor (or equivalently, the pair distribution function) of colloidal fluids is a non-analytic function of the shear rate. The Smoluchowski equation shows boundary-layer behaviour at zero wave vector (large separations). The width of this boundary layer is proportional to the square-root of the shear rate. An explicit perturbation analysis of the two-particle Smoluchowski equation without hydrodynamic interactions is given. The onset of the non-Newtonian behaviour of the effective vicosity is due to the boundary-layer behaviour of the structure factor.