According to Graphtec's website, they don't sell Nubone blanks:

"Why is NuBone only available in pre-slotted and compensated models?

The reason we only make per-shaped NuBone nuts and saddles is that if you are going to spend the money on a custom shaped nut or saddle you might as well use one of our premium TUSQ product because most of the cost is in the labor."

So in theory if you want a Graphtech blank nut, you'll need to use TUSQ nut which is harder and a bit brighter sounding than Nubone.

Contrary to their statement, Graphtec do sell unslotted Nubone nut blanks to suit various guitar/bass types, but only in packs of 10.

As a nut material, I don't know enough to make comments. I think I've got some guitars with Tusq XL nuts, but I never like them that much as the graphite in them makes them hard to clean and polish, the slots always look dark and messy. So I've often replaced them with standard bone nuts.