Use wallpaper tool, select color and chose black.
That's the first thing tried, it does not remove the TC logo, and it does not make the wbar black either. In the end wbar seems to only get transparent if you put "-balfa 0" at the end of the one line in ~/.wbar. Got the logo to disappear too in the end, fingers crossed, by commenting out the hsetroot line in ~/.setbackground.
Perhaps such hacking could be unnecessary if the wallpaper app supported disabling the wallpaper and if the wbar defaulted to transparent.