The black stain is caused by the number of raytrace bounces being higher than the limit in your render settings. You've got two choices. You can turn up the raytrace bounces render setting, or you can use some sort of shader in the background colour input of the refract node to replace the black with something that blends in better. Turning up the bounces will slow down the render, so don't set it too high. I find 10-12 usually works pretty well.

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