Most played cards in Mono Black:

4 Decks
Duress
14 copies - 3.5 average
Cabal Stronghold
12 copies - 3 average
Ravenous Chupacabra
12 copies - 3 average
Vraska's Contempt
12 copies - 3 average
Doom Whisperer
11 copies - 2.75 average
Karn, Scion of Urza
10 copies - 2.5 average
Field of Ruin
10 copies - 2.5 average
Kitesail Freebooter
10 copies - 2.5 average
Ritual of Soot
9 copies - 2.25 average
Plague Mare
8 copies - 2 average
Plaguecrafter
8 copies - 2 average
Dusk Legion Zealot
8 copies - 2 average
The Eldest Reborn
8 copies - 2 average
Moment of Craving
7 copies - 1.75 average
Chromatic Lantern
6 copies - 1.5 average
Find // Finality
6 copies - 1.5 average
Dead Weight
6 copies - 1.5 average
Cast Down
6 copies - 1.5 average
Walk the Plank
5 copies - 1.25 average
Midnight Reaper
4 copies - 1 average