I agree, I think 8 would be too much and you would lose the block pattern that way. Using two colors per block would still be scrappy since every block will have a different 2 colors. I too would use light and dark as shown on the pattern since this will again highlight the pattern. You want to be able to see what the shapes are and I think if you do dark on dark, you won't see that.
Have fun! Can't wait to see it!!
Rachel