From a Google search:
the be-all end-all of C4 ZR1's! Originally developed to be the successor to the LT5, the "Black Engine", as it came to be known, was produced by Mercruiser, and then sent to Doug Rippie Motorsports (DRM) for reassembly and tuning. The Black Engine included reworked camshafts, cylinder heads, intake, and throttle body for a hefty 70hp gain over even the higher-powered 1993-95 ZR1's. DRM topped off this package with the newly developed Borla catback exhaust system, high-performance clutch, coil-over shocks, and other minor suspension modifications. The engines and intake manifolds were then refinished in a black crinkle finish, hence the name Black Engine. Unfortunately, lagging sales spelled out the end of the C4 ZR1, and just 8 ZR1s (all of which already had owners) were fitted with these modifications.
Engine made 475HP. Never heard of it before.