Creatures
Blue tends to have the lowest creature count of any colour in Cube; its primary contribution to aggressive decks is disruption. However, as Magic in general becomes more creature-focused and themes in recent sets push all colours in that direction, a more creature-heavy blue theme is starting to look more viable.
The creatures that blue does have tend to be big threats on their own or have an effect that makes them worth playing – Elite Vanguards and Goblin Guides aren’t in blue’s remit. Most of its aggressive
(or ‘tempo-oriented) creatures have a type of evasion, usually flying but sometimes unblockability.
Counterspells
Blue’s access to cheap, all-purpose answers in the form of countermagic is one of its main attractions. They’re useful both for stopping anything that might hinder your army as well as buying time for your more expensive cards to come online.
Bounce
Blue also has ways to deal with cards that hit the board. These are only temporary answers but they buy you time, and that’s often all blue needs. These are at their best when they are efficient, delaying more expensive plays for only a few mana; hence blue’s weakness to cheap cards and aggressive strategies.
Bounce can also be used in a pinch to save your cards from removal or to reuse enter-the-battlefield triggers.
Stealing
These are some of blue’s strongest effects. They’re more vulnerable than bounce, as most of them are conditional on the card staying in play. Even so, the ability to make your opponent’s best card work to your advantage – often leaving them down a card in the equation – is tremendously powerful.
Card Draw and Selection
Blue not only has strong cards, but gets to draw more of them. The threat of being overwhelmed by card advantage causes opponents to play into blue’s countermagic or mass removal provided by other colours.
Artifacts
One strength of blue is that it interacts well with artifacts, the ‘community cards’ of Cube. This ranges from getting a little value with Trinket Mage to terrorizing the opponent with an early Sundering Titan
or Myr Battlesphere via Tinker.
Big Discounts
Many of the game’s ‘mistakes’ and splashiest cards can be found in blue. Cards like Tinker and Show and Tell have been menaces in Constructed formats, and are just as good here when you get to ensure the Cube has enough targets.
Combo
If playing fair isn’t your thing, blue has what you’re looking for. Combo of this sort requires a large commitment, and maybe isn’t advisable if you’re building your first Cube, but the tools are there if you want to go this route.
Looting
Much of blue’s card draw requires discard as compensation; most of the time this is a downside, but with the wide range of graveyard effects in Cube you can turn it to your advantage. Blue doesn’t have many cards that interact with the graveyard, but this provides a natural bridge with colours like green or black that can make better use of it. Reanimator, for instance, values cards like these very highly.
As you can see, blue’s bread-and-butter effects are mostly instant-speed, giving a blue player a great deal of flexibility and forcing the opponent to play around many different cards. Blue is often favoured by experienced players, as it allows them to leverage their skill as well as prompt and capitalize on errors.