Yeah, I've seen a lot of classifications. Surgical black, true black, "blacks" that aren't black, true hard, black hard, and so on. That's part of what makes the guess work difficult. Sidmind's guess - hard black - sounds like one I'm familiar with, but can't be the same as the 'black hard' classification I know of because that only comes in 2 colors, black and blue-black.

The discolored spot looked to me like someone got a drop of oil on the edge and cleaned away the swarf there. But that may be natural discoloration. In any case, if that turns out to be a finishing grade Arkie, I will be ecstatic. I can't imagine anyone needing a medium/hard grade stone that size (12" x 2.5" x 1") but stranger things have happened.