Blackie symbolizes the more typical boys' gang leader of the early part of the twentieth century. Like the other boys who join together in mischievous conduct for thrills and a sense of camaraderie and belonging, Blackie is one of the group, and is a friend of the other boys. The acts that he and the others commit are emotionally driven whereas Trevor--T. as he is called because he has an upper class name--are simply acts of nihilism, symptomatic of the psyche of many a Londoner after the destruction of World War II.
"All this hate and love," he [T.] said, "it's soft, it's hooey. There's only things, Blackie," and he looked round the room crowded with the unfamiliar shadows of half things, broken things, former things....
Whereas T. perceives destroying Old Misery's house as a creative act (for which he exploits others in his design) since it is performed craftily so that it does not collapse until nothing inside is left, Blackie and his gang have entered Old Misery's home before and committed single acts of vandalism without stealing any of his money or destroying the house. T. does not want the money either, but he has it burned, again enjoying pure destruction.
The only reason that Blackie joins in the tearing down of Old Misery's house is the fact that he wants to be with the gang and he desires fame for his friends:
Driven by the pure, simple, and altruistic ambition of fame for the gang, Blackie came back to where T. stood in the shadow of Misery's wall.
Thus, Blackie symbolizes a pre-World War II gang leader, a leader in joint vandalism and pranks as acts of bravado and camaraderie. This type of leader is unlike the nihilistic Trevor/T.