Most played cards in Mono Black:

4 Decks
Dark Ritual
16 copies - 4 average
Underworld Dreams
16 copies - 4 average
Sinkhole
16 copies - 4 average
Hypnotic Specter
16 copies - 4 average
Mishra's Factory
16 copies - 4 average
Terror
14 copies - 3.5 average
Black Knight
12 copies - 3 average
Gloom
12 copies - 3 average
Drain Life
11 copies - 2.75 average
Juzam Djinn
11 copies - 2.75 average
Paralyze
9 copies - 2.25 average
Nevinyrral's Disk
8 copies - 2 average
Sengir Vampire
8 copies - 2 average
Urborg
4 copies - 1 average
Juggernaut
4 copies - 1 average
Hymn to Tourach
4 copies - 1 average
Strip Mine
4 copies - 1 average
Black Lotus
4 copies - 1 average
Mind Twist
4 copies - 1 average
Artifact Possession
4 copies - 1 average