Most played cards in Mono Black:

1 Decks
Smallpox
4 copies - 4 average
Gravecrawler
4 copies - 4 average
Withered Wretch
4 copies - 4 average
Tragic Slip
4 copies - 4 average
Bloodghast
4 copies - 4 average
Viscera Seer
4 copies - 4 average
Geralf's Messenger
4 copies - 4 average
Nykthos, Shrine to Nyx
3 copies - 3 average
Duress
3 copies - 3 average
Funeral Charm
3 copies - 3 average
Gray Merchant of Asphodel
3 copies - 3 average
Erebos, God of the Dead
2 copies - 2 average
Extirpate
2 copies - 2 average
Mutilate
2 copies - 2 average
Relic of Progenitus
2 copies - 2 average
Whip of Erebos
2 copies - 2 average
Phyrexian Arena
2 copies - 2 average
Thoughtseize
1 copies - 1 average
Relentless Dead
1 copies - 1 average
Ruinous Path
1 copies - 1 average