Darkness did have an unusual vast knowledge of surgebinding. As Lift is sliding everywhere, he comes to the conclusion she is an Edgedancer, saying, "It appears you are an Edgedancer... I had wondered which of the two you would be. This shows he has some inside knowledge to surgebinding, knowing it comes in pairs that signify different orders. He then relates,
“They were once a glorious order,” Darkness said, walking her down the hallway. Everyone ignored them, focused instead on Gawx. “Where you blunder, they were elegant things of beauty. They could ride the thinnest rope at speed, dance across rooftops, move through a battlefield like a ribbon on the wind.”
Finally, it makes sense he belongs to sky breakers as justice is the only thing he cares about. He also is shown to have a shard blade here.
“Why...why do you hunt me?” (Lift speaking)
“In the name of justice.”
“There are tons of people who do wrong things,” she said. She had to force out every word. Talking was hard. Thinking was hard. So tired. “You...you coulda hunted big crime bosses, murderers. You chose me instead. Why?”
“Others may be detestable, but they do not dabble in arts that could return Desolation to this world.” His words were so cold. “What you are must be stopped.”
“I saved him,” Lift said. “I did something good, didn’t I?”
“Goodness is irrelevant,” Darkness said. His Shardblade dropped into his fingers.