Most played cards in Mono Black:

3 Decks
Dark Ritual
12 copies - 4 average
Underworld Dreams
12 copies - 4 average
Hypnotic Specter
12 copies - 4 average
Sinkhole
12 copies - 4 average
Mishra's Factory
12 copies - 4 average
Terror
11 copies - 3.67 average
Gloom
9 copies - 3 average
Drain Life
9 copies - 3 average
Black Knight
8 copies - 2.67 average
Juzam Djinn
7 copies - 2.33 average
Paralyze
6 copies - 2 average
Sengir Vampire
6 copies - 2 average
Nevinyrral's Disk
5 copies - 1.67 average
Artifact Possession
4 copies - 1.33 average
Juggernaut
4 copies - 1.33 average
Order of the Ebon Hand
4 copies - 1.33 average
Hymn to Tourach
4 copies - 1.33 average
Su-Chi
3 copies - 1 average
Urborg
3 copies - 1 average
Mind Twist
3 copies - 1 average