Well, I think since Black hat SEO means using unorthodox methods to do SEO and White is doing the exact opposite. The difference between Black hat SEO and Grey hat is the fact that Grey hat has to do with finding a middle ground between Black hat and White hat; In Grey hat you're not really all that saint or different from a Black hat but you're treading carefully by making use of a shade of grey that SE might never get to see that you're not really what you are.
IMO, the difference between the two is Grey hat is a less-selfish act compared to Black hat. And you know what they say about not being greedy? You last.