I have this happen sometimes with darker clay bodies. As was said above, it's usually scumming; but it can also be dust / detritus loose in the kiln from pieces from a white body, or even kiln wash if you're unlucky.
I'd say about 80% of the time, I can fix it with some vigorous sanding in the bisque stage. Obviously take proper precautions to not breath the dust from sanding.
If this doesn't work, then your best bet is to cover it up. It can absolutely show in the final firing.
Neil, yes the inflated rubber wheels DID seem odd to me... I'd much rather have a set of hard castors or something shielded from the heat somehow.
And I'm not worried about size. Smaller is actually probably better for me in a city studio (to a point). But at $6700 you could definitely be looking at 12-ish cu ft front-loaders... If I could house them.
And that's a good note about the chimney's bagwall too. I was curious about that. We've all seen kiln shelves warp over time, but I thought it was more from gravity while they are hot (a sagging effect). I didn't know they'd warp in a vertical position. Good to know!