Actually black territories on a black background isn't a problem. When you upload the SVG, anything marked as a territory will show up as white. If nothing shows up, your most likely problem is that you haven't defined the territories as territories.
) section of the wiki to try and solve your issue. If you're still having problems, what I did was open a new InkScape file and practice by uploading small, simple shapes to the WL map designer until I understood how it works.