An ATC Local Controller reported to a departing aircraft that its right engine was producing black smoke. The crew returned to land and was told by the runway safety crew that smoke and leaking fluid was coming from the engine. Maintenance found a fuel leak.

Narrative

After TOGA takeoff at approximately 400 FT; ZZZ Tower advised us that we had black smoke from our right engine; and asked our intentions. We did not have any cockpit indications of abnormalities. This was a busy portion of flight with clean-up and compliance with SID. ZZZ Tower again asked our intentions and we stated we wanted to stay on SID and level at 5;000 FT. We switched to Departure Control and stated we would be returning to ZZZ for a precautionary landing and that we were in no apparent danger. We did not declare an emergency and indicated emergency equipment was not needed. First Officer talked to Flight Attendants and sent a message to Dispatch that we were returning to ZZZ. I talked to passengers and explained the same. We checked landing weight and required runway length and determined that Runway 4 was adequate for our landing weight of 170;500 LBS. Gear and flaps were extended early to facilitate fuel burn. Normal landing and exited at Taxiway G. After clearing runway safely; crew stated right engine was still smoking and there was a fluid leak. We shut it down and fluid leak subsided. Taxied to gate and was met by Maintenance. Callback conversation with Reporter revealed the following information: The reporter stated that maintenance opened the engine cowling and during an engine run up discovered a fuel line O-ring leaking where it entered the fuel control. The engine was shut down and the aircraft removed from service.