Also, you might try increasing the "quality" integer on the refraction node. I don't know what your settings are, but that could have an impact. Also, if your refraction background is black, try changing it to a lighter grey color and see if that helps. Those black spots look like render artifacts to me. Basically, from having a low quality setting. Black areas of refracted material can come from light rays being bounced around, and not "escaping" the material. Be warned, increasing the refraction quality will also increase render time.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.