Most played cards in Mono Black:

2 Decks
Sign in Blood
8 copies - 4 average
Chittering Rats
8 copies - 4 average
Cuombajj Witches
7 copies - 3.5 average
Thorn of the Black Rose
6 copies - 3 average
Choking Sands
6 copies - 3 average
Barren Moor
5 copies - 2.5 average
Rancid Earth
4 copies - 2 average
Gray Merchant of Asphodel
4 copies - 2 average
Duress
4 copies - 2 average
Befoul
4 copies - 2 average
Icequake
4 copies - 2 average
Dark Ritual
4 copies - 2 average
Phyrexian Rager
4 copies - 2 average
Geth's Verdict
4 copies - 2 average
Peat Bog
4 copies - 2 average
Gurmag Angler
4 copies - 2 average
Diabolic Edict
4 copies - 2 average
Pestilence
3 copies - 1.5 average
Wrench Mind
3 copies - 1.5 average
Crypt Rats
3 copies - 1.5 average