DBC doesn't have alpha channel support, just limited transparency capabilities.
Set Object
Set Sprite
Ghost Object On
Set Object and Set Sprite allows you to make all pure black colors (RGB(0,0,0) or hex 000000) transparent. To make a black color appear, use a color that is near black but isn't pure black (like RGB(16,16,16)). It's limited to black only though, so don't expect varying alpha transparencies.
Ghost Object On is a little different. It enables alpha blending on the object. The darker a color, the more transparent it becomes. However, there is an optional parameter to enable "dark ghosting" which inverts the blending mode.
Try it all and you'll see what I mean.