The Chapter Black tape is an artifact from the Spirit World which is the focal point of the Chapter Black Saga. Originally kept in the deepest bowels of Spirit World's Records Department, it is a compilation lasting for thousands of hours, said to contain the atrocities that human-kind had committed over millennia of years. The tape is so legendary, it is known to demons such as Kurama and Hiei, so much so that Hiei himself sought to acquire the item. According to Koenma, the Chapter Black tape is part of a set which is never meant to be seen alone.  Therefore, its logical to assume the companion tape is called Chapter White; it would contain the good humanity has done.
The tape's contents are so horrifying that it can turn humans against their own kind such as the case with Kiyoshi Mitarai, where his thoughts were so warped, that he considered himself to be the perpetrator of the atrocities found in the video, and is considered justifying evidence that demons seek to legitimize their cruelty and atrocities on humankind. More severe cases include Dr. Minoru Kamiya, becoming absolutely nihilistic and emotionally unstable to the point of homicidal madness, and Spirit Detective Shinobu Sensui, whose period of moral and ethical disenchantment was soon driven into absolute misanthropy and a desire to end the human race.
It vanished from the Spirit World Records Department vault along with Shinobu Sensui after he ended his tenure as a Spirit World detective ten years before the events of the series.
After the events of the Chapter Black Saga, Hiei acquires the Chapter Black tape and after Kurama inquires what Hiei will do with the tape, he destroys it. 
- Within the context of Yu Yu Hakusho's themes of the paranormal and the supernatural, the concept of the Chapter Black tape is likely derived from pre internet era mythos and urban rumors of physical media kept underground from public consumption or is kept adrift in obscurity due to its questionable and shocking content. Considering its more inhumane and evil aspects, the Chapter Black's contents are likely derived from the concepts of recording real life atrocities for the purpose of entertainment, including snuff films and personal mementos of serial criminals.