Black Heart was an individual who lived centuries or even thousands of years ago, an infamous terrorist who spread his trade across the galaxy in the Alpha and/or Beta Quadrants.
In the 23rd century, a Starfleet officer (unnamed USS Enterprise-A personnel) had visited Kreel, where he heard tales of the Worthy's encounters with Black Heart. When the Federation starship USS Enterprise-A rescued survivors of the ancient Worthy expedition in 2288, the officer asked Catalano, the Karimean leader of the Worthy, if he remembered the encounter. Catalano began to answer, but began to feel self-conscious about relating the tales after mentioning Black Heart's death at his hands. Catalano's initial memories were of Black Heart's genocidal decimation of Kelikeli with plague disease, the toxic war he incited against Jaunya III, and the pointless destruction of the Holy Coral Gardens of Spungka'a. After Eyleen suggested that it might be entertaining to hear more, she brought up an occasion where the Durlan of the Meganites contacted the Worthy with a distress call and led to Catalano being able to thwart Black Heart's bid to steal the Meganite crystal purifier. (TOS - The Return of the Worthy comic: "Great Expectations!")