Blue lacks an evergreen creature combat keyword to call its own, and needs one. That's a well-known current design problem, along with a lack of an appropriate crossover keyword for blue and black and especially for blue and red. The reasons for this are simple: Combat is the most interactive part of Magic, and reducing opponents' life totals to zero is the most usual victory condition. Not letting blue participate in this as much only incentivizes the color getting degenerate uninteractive designs and strategies. Blue's only real combat keywords tend to be evasion, which is uninteractive by nature. While R&D wrestles with this problem still, the custom card designers of MTGSalvation pooled their own ideas for a blue combat keyword. Here are some of them:
Combat charmer (Spells cost you 1 less to cast as long as this is attacking or blocking.)
My own suggestion. Pros: Lets blue do cool stuff sooner, feels very intelligent. Cons: Does nothing without outside cards (i.e. instants/flash permanents), maybe too good in multiples.
Dispel (Creatures blocking or blocked by this creature use only their base power and toughness this combat)
Pros: Simple, clean, new design space, feels "tricky." Cons: Narrow, pure hoser, reactive rather than proactive (does nothing against unpumped creatures), unintuitively kills base 0/0s with +1/+1 counters.
Scouting (Whenever this creature becomes blocked, you may untap it and remove it from combat.)
Pros: Makes for interesting decisions. Cons: Probably too game-slowing, leads to less happening rather than more.
Backstab (Whenever this creature attacks a player and isn't blocked, you may have it assign its combat damage to target creature that player controls.)
Pros: Very interactive. Strong and wow-inducing. Can be put in black or red too. Cons: Repeatable creature removal is so strong that this might be unkeywordable. Would need to be costed very conservatively. Hard to justify mechanically in blue without stretching. Yes, technically it's direct damage by combat, which ALL colors have access to.
Taunt (At the beginning of combat on each opponent's turn, you may untap this creature and target creature that player controls. The targeted creature attacks you if able and this creature blocks it if able.)
Pros and cons: See previous entry.
Guile (Prevent all damage that would be dealt to this creature by creatures with greater power or toughness than it.)
Pros: Add survivability to maligned blue small creatures. Very flavorful. Cons: Encourages board stalls and chump blocking.
Misdirect (You may pair this with another unpaired attacker as they attack, until blocking assignments are finished. Then you may switch those assignments.)
Pros: Workable way to do the very old blocker switching idea. Super tricksterish. Lots of fun to keep your opponents on their toes. Cons: Like combat charmer, does nothing for the creature on its own.