I model in Silo2. When I import some objects (clothes) into Poser, there are sometimes dark patches. I have tried everything I can think of to remove them. They seem to be something to do with smoothing as they look okay in Silo. In UV mapper they also appear. This usually appears when there is a "hem" on the clothes as in this picture.

Can anyone suggest anything to improve the models?

That's a mighty subtle effect.  I had to color-pick in Paintshop to be sure the darkness wasn't just an optical trick caused by the red square.

On a hem, the front and back layers are sometimes too close together for Poser to handle.  Poser doesn't like two opposite-facing facets that occupy the same location.  But usually this problem yields a checkered or herringbone appearance, not a vaguely darker area.

Thank you very much, Ockham. I had two circles of faces on the edge and pulled the ring of vertices in the middle of those down a fraction. For some obscure reason, that seems to have done it, so it does sound as though the slight difference in the angles of the faces was the problem. I will take note of your advice in future! 

P.S. Love that avatar.