My first thought would be to darken the diffuse color way down, but I don't have experience with this particular use case to know if it'll do what you want.
@perna: well no, it represents how tinted the specular color is by the diffuse. i interpolate between white and the diffuse color, based on this tint value. so mix(white, diffuse, tint).
You have your diffuse color set to black and your diffuse Texture is plugged in the reflective input. For metal materials you need a dark diffuse color but not for non metals.