A truly black background would have reflected no light onto the underside of the product, so your goal in photoshop would be to reduce the amount of brightness underneath
Since you already have a nice mask of the product, I would begin by duplicating the layer, offset it by some amount - right where the shadow in the tool begins to go from dark to light again (caused by the reflection of the white surface). Then work with a Curves or Levels adjustment layer to adjust the brightness of that duplicated dark layer.
You will also need to create a positive mask, whereby you subtract the top layer from the lower layer, in order to allow the original image to dominate.
By working with three or four layers, and a positive mask you could get the product to look like it was shot against a black background in a reasonable amount of time. It won't look perfect however, because of the nature of this 3D object captured in 2D, there will be a visible seam. So to get even closer to reality, I would break each part out with its own individual mask, and offset amount. One mask each for the rubber handles, one each for the orange arms, probably one for the bolt cutter section. Essentially repeating the steps above for each individual part of the product.