The soft black edges are known as vignettes. All that is happening is that edges of the converter are being 'seen' by the sensor. Some lenses get this even with just a UV filter on.
You can try to get rid of vignettes by stopping the lens down (i.e. from f4 to f11) but that doesn't always work - it depends on how much of the converter is in the field of view of the lens.
Hope that helps.
a900+VG + A700
CZ24-70, Sony 70-200G, Sony 70-300G, Sony 135 STF, CZ85mm, KM 20mm, KM 24mm RS, KM 17-35G, KM 400/4.5G HS, Tamron 90mm Macro, Sony 50 1.4, Sony 16mm Fisheye
Sony HVLF58AM, HVLF42AM
Take nothing but pictures, leave nothing but footprints.