I tried the bluish sheen, tried reflective map. Lighting seemed to be helpful but infinite light made the white blue. This is a challange, but I feel that the trick is to get closer to how poser "computes" light reflection when the color is "black" There is very little reflection if the underlying texture map is all black. I am still working on this, but there are two types of "black" horses, one that is called true black and a seal brown which is mostly black with brown highlights. True black is defined as being black with no brown, although there are usually a few white flecks, there is usually no brown otherwise it is a brown or seal brown. I noticed a change with the reflection map, though I don't know why it should change it did. Using all white reflection map on the dark grey with the settings as above gives a bit better sheen to the coat.