If you ask about 3D, you can use -set object - command where you can set transparency flag to 0 and all parts of object textured black will be invisible.
Black means rgb walue (0,0,0) to rgb(10,10,10) everything else will be displayed.
If you mean semitransparent then you can use -ghost object- command. In this case hovewer everything on object will be semitransparent regardles on color, but darker colors will be more transparent then lighter ones. Black color will be in fact totaly transparent.
Try these comands and see what they do.