I find that when I ride on cement, especially when I’m doing snake shapes (think of turning around a line of cones, in out, in out, you get it) I make terribly noticeable black marks. I don’t know if this is just from a cheap tire or am I just grinding my tire into the pavement too much when I do stuff like this?
Sounds like you need more air in your tube.
It’s only 5 PSI below max (60 I think it is)
I leave black marks on the pavement wherever I go too. I ride a 700x23c tire pumped up anywhere from 80-120psi. I’ve left marks with every tire I’ve run on every one of my unicycles, but I never leave marks when on my road bike. I must conclude it’s something to do with the unique stresses unicycling places on the tire.
I actually find it quite hilarious…I always ride past the same stoplights, and all the poles I hold on while waiting for said lights have copious permanent black tire marks from me 
I love the marks actually. Its funny, you go to my skatepark and all the biggest rails/gaps are totally clean EXCEPT! from marks from my uni.
God, I own my town ^.^
sort of related topic (sorry for the threadjack): one place where I ride passes through a sand/gravel parking lot that sees some traffic from hikers, mountain bikers etc… so I love to trace out shapes in the lot i.e. loops and tight figure eights if for no other reason it confuses the heck out of folks trying to figure out what made those shapes! Offroad unicycling is practically unheard of around here…
If I do really sharp turns, I leave black. I don’t like doing that. It wears out my tire.
Slightly off topic: The word of the day from Wordsmith is “snake eyes.”
MEANING:
noun: A throw of two ones with a pair of dice. Since this is the lowest possible score, by extension the term is also used to refer to bad luck.
As you say, all to do with unicycles putting lots of torque through that tyre when pushing down hard on the pedals. Not something that happens the same with bicycles - unless maybe the fixie crowd say otherwise.
Well if you all do it too, then I guess I’m doin somethin right 
I’ve left a few pretty cool twisty shapes in friends driveways.
Leaving black marks is one reason why you should be careful about riding on conspicuous public property, i.e. the courthouse steps, concrete picnic tables at the park, etc.