I have to say I've never seen black urates... or gray ones, for that matter. My snakes always poop out nice white or yellow urates.
If it doesn't look normal (as compared to your other, presumably healthy snakes), then it probably isn't normal, and you should take it to the vet. Whichever way, something doesn't seem right - poops should be poop colored (or the color of the rodent), with nice white urates. If the urates aren't white, then the kidneys (the organs that produce pee) are filtering out something that is staining the urates grey/black, and I'd take my snake to the vet to find out what.
And in the unlikely case it's actual calcium deposits...that's not normal either and the snake should see a vet! 
~jenny
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1.2 normal ball pythons (Cindy, Darwin, and Periscope)
0.2 rescue chinese water dragons (Yoni and Linga)
1.0 rex rat (Scurvy)
1.0 gerbil (Yerbul)
0.1 bunny (Spazz)
1.1 betta fishes (Vicious and Killer)
2.2 great danes (Shasta, Odysseus, Merlot, and Watson)
1.0 fat fuzzy mutt (Smokey)
1.1 cats (Thidwick and Turtle)
3.0 horses (Buddy, Sam, and Scout)
1.0 goat (Billy Jack)
1.25 chickens (Ugly the rooster and his harem)
