The Black Sea was a sea on Earth. This sea at night was always a stimulating experience for Worf.
In 2370, aboard the USS Enterprise-D, Worf and Deanna Troi went on a date in the holographic program Black Sea at Night. They were walking barefoot together on a beach along this sea listening to balalaika music. (TNG: "All Good Things...")
Retroactive continuity; A 20th century-era statistical map of Europe, Africa, and Asia was stored in the USS Enterprise library computer in 2254. On this map, a body of water, located between Western Asia and Eastern Europe, was labeled as the "Black Sea". This map was among the materials viewed by the Talosians when they scanned the Enterprise computer. (TOS: "The Cage")