Most played cards in Mono Black:

4 Decks
Archfiend's Vessel
14 copies - 3.5 average
Agadeem's Awakening
8 copies - 2 average
Call of the Death-Dweller
8 copies - 2 average
Grasp of Darkness
8 copies - 2 average
Whisper Squad
8 copies - 2 average
Serrated Scorpion
8 copies - 2 average
Duress
8 copies - 2 average
Kitesail Freebooter
8 copies - 2 average
Heartless Act
7 copies - 1.75 average
Murderous Rider
6 copies - 1.5 average
Tymaret, Chosen from Death
6 copies - 1.5 average
Village Rites
6 copies - 1.5 average
Skyclave Shade
6 copies - 1.5 average
Bastion of Remembrance
4 copies - 1 average
Mire Triton
4 copies - 1 average
Lurrus of the Dream-Den
4 copies - 1 average
Underworld Dreams
4 copies - 1 average
Lampad of Death's Vigil
4 copies - 1 average
Gray Merchant of Asphodel
4 copies - 1 average
Hunted Nightmare
4 copies - 1 average