Most played cards in Mono Blue:

5 Decks
Serendib Efreet
20 copies - 4 average
Psionic Blast
19 copies - 3.8 average
Counterspell
19 copies - 3.8 average
Mishra's Factory
16 copies - 3.2 average
Control Magic
16 copies - 3.2 average
Blue Elemental Blast
16 copies - 3.2 average
Juggernaut
12 copies - 2.4 average
Su-Chi
12 copies - 2.4 average
Unstable Mutation
10 copies - 2 average
Flying Men
8 copies - 1.6 average
Strip Mine
8 copies - 1.6 average
Dandân
8 copies - 1.6 average
Hurkyl's Recall
8 copies - 1.6 average
Black Vise
7 copies - 1.4 average
Energy Flux
7 copies - 1.4 average
Maze of Ith
6 copies - 1.2 average
Chaos Orb
5 copies - 1 average
Ancestral Recall
5 copies - 1 average
Sol Ring
5 copies - 1 average
Time Walk
5 copies - 1 average