The Black Raven has a very lanky, tall build, and wears long black robes. He has a bird-like head, with furry 'ears'. He wears a necklace.
Black Raven is the disguise used by the children of the Black Ravens, headed by their leader, Crow.
They confuse visitors with deceptively quick movements and set puzzles to test those who seek entry to the black market.
In Last Specter, Crow is the head of the black market. The children who run the black market (the Black Ravens) find things in rubbish dumps and alleyways, then clean and mend them before auctioning them off on the market. Some things they find (such as Arianna'socarina, which was found in a rubbish pile according to Louis) hold real value, but most of it is just junk. Nevertheless, the black market is very successful. There are 9 Black Ravens; Crow, Marilyn, Nabby, Scraps, Tweeds, Wren, Socket, Louis and Badger.
The Black Raven appears early into the game as a mysterious and antagonistic character. He leapt down from a rooftop onto a stall Professor Layton, Luke, and Emmy were browsing. Emmy proceeded to chase the Raven in a parkour fashion until he vanished completely. He stopped momentarily to tell the three that they must "complete [his] body" in order to become members of the black market, which was a hint to them to find the medals.
The Raven and the black market turned out to be the idea of a group of schoolchildren led by a boy called Crow. The Raven was a way of testing the potential member to see if they were worthy. In order to give the impression of moving incredibly quickly from place to place, the children dressed up as several Ravens, each trading places with the other at certain intervals.
Later on, the Black Ravens help Layton twice. The first time, Layton needed help with setting up traps for the Specter around Misthallery, so he called upon them. When Descole revealed himself and set his Specter machine to destroy the town, the Black Ravens helped Layton collect parts for and build a catapult, which temporarily took out the machine.
What's that underneath the Raven's robes?
The members of the Black Raven, with Crow shadowed.