This Speedotron 40" Umbrella with Removable Black Backing gives you two great options. You can bounce your light source off of the white interior with the black backing on to reduce loss of light through transmission. Alternatively, you can remove the black backing and direct the light through the umbrella and onto your subject as is done with a softbox.
In the Box
Speedotron 40" Umbrella with Removable Black Backing