I have the same problem. Textures that include ‘special’ bitmaps appear deep black. The problem appeared on my old iMac and I thought that it could be a problem with the virtual machine but then I took my new Lenovo Yoga 920 where die textures are black, too. I experimented a little bit and found out, that some bitmaps work fine but others not although I could not find any difference in the file formats. Thus, when creating a new user defined material with a color texture that appears black you can solve the problem checking the color mask bottom. Then it works but it is not the solution. When using plugins like Shapediver for Grasshopper which address the Rhino viewer there is no way for checking this mentioned box. Maybe it would work if the box is checked by default.
Here some screenshots:
This shows the problem in rendered view.
This shows the problem in shaded view with only the object being rendered.
Settings with unchecked box for color mask.
Settings with checked box for color mask.
Another material with another bitmap that works. Both are pngs with 24 bit. I also tried to convert the images with and without alpha cannel or into jpgs without any success.
I use the latest Rhino release candidate. Both textures are located on my dropbox. The problem also appeared when the bitmap is saved directly on the desktop.
I mentioned Shapediver… It is interesting, that shapediver is giving back also only black textures in Grasshopper respectively in the Rhino viewer but uploaded at Shapediver the textures appeared completely well.
Hoping for a solution.