The VTC solenoids do have a rubber ring around the back but the center is just a black goop. Unless the rubber around the edges has melted then I would think its just the goop coming off.
1989 240SX Fastback. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1995 300ZX Twin Turbo Warning: Objects in mirror aren't as fast as they thought they were.