does anyone know if there is a large, fairly decent quality, 3x3x3 cube available in black?
Not yet. You can make one by your own:
a) Get a foot cube and dye/paint it black.
b) Use a 6x6x6 and glue pieces toghether.
c) Also you can extend the pieces of a normal 3x3.
More ideas to be posted soon!(I have a hard time thinking)