Most played cards in Mono Blue:

2 Decks
Serendib Efreet
8 copies - 4 average
Counterspell
8 copies - 4 average
Psionic Blast
7 copies - 3.5 average
Blue Elemental Blast
7 copies - 3.5 average
Unstable Mutation
6 copies - 3 average
Strip Mine
5 copies - 2.5 average
Flying Men
4 copies - 2 average
Psychic Purge
4 copies - 2 average
Mishra's Factory
4 copies - 2 average
Dandan
4 copies - 2 average
Juggernaut
4 copies - 2 average
Hurkyl's Recall
4 copies - 2 average
Su-Chi
4 copies - 2 average
Energy Flux
4 copies - 2 average
Control Magic
4 copies - 2 average
Old Man of the Sea
3 copies - 1.5 average
Tolaria
2 copies - 1 average
Mana Vault
2 copies - 1 average
Copy Artifact
2 copies - 1 average
Boomerang
2 copies - 1 average