At this year’s block party on 136th and Broadway in Harlem, a trio of graffiti artists took over a white wall as the cops looked on. Well, okay, it was more a giant piece of butcher paper than a “wall,” but these three really transformed it into a mural commemorating the afternoon’s festivities. The two younger artists were guided by the older graffito, who acts as a sort of sensei, not unlike Lady Pink’s mentoring of young artists in Queens. But this mural is definitely a collaboration. It was sponsored by a group called Xmental, a group of “community organizers, artists, teachers of hip hop arts and sustainable education,” who lead classes for kids convicted of vandalism.