If your falloff is at zero, and your highlight is black, then perhaps it is the transmap itself. This look can also happen if the dark color of the transmap, the part that is supposed to make things transparent, is not true black. Take the transmap into an image editor, and use the eyedropper tool to select the darkest color. Now look at the hexidecimal value of that color. It should read #000000, HSL color should also be 255, 0, 0; and RGB, should be 0, 0, 0. anything else will result in a faulty transmap.