Blue is one of the strongest colours in Magic The Gathering's Commander format. The colour of card draw and counterspells, blue players are feared and untrusted in most games they sit down at. Players will always be afraid of any mana you leave open, even if you're playing something as wholesome as Merfolk tribal.
The enchantments blue players have access to really don't help with that uneasiness. Some of the best enchantments in the entire game are blue cards, which can majorly tilt your games' power balance in your favour. Here are the best blue enchantments you may want to consider running in your EDH deck.
5 Reconnaissance Mission
Blue is by far the best colour at hand advantage – the more cards you can draw, the more options you have. Sitting there with a grip of 20 cards and enough mana to keep your opponents at bay is an excellent feeling, and Reconnaissance Mission is a fantastic card for it.
Reconnaissance Mission costs two generic and two blue, and allows you to draw a card whenever a creature you control deals combat damage to an opponent; hit with three creatures, you get to draw three cards, and so on. If you're already sorted for draw and don't need it, Reconnaissance Mission also has a cycling cost to let you discard it and pay two generic to draw a new card instead.
It may seem odd for blue to base an effect on dealing combat damage, when that's usually more of a red and green thing. But blue is also a colour with many evasive creatures, particularly ones with flying. It may not have the biggest attackers in the game, but blue is great at slipping past defences to ensure you're drawing cards off Reconnaissance Mission.
4 Leyline of Anticipation
In Commander, anything you can play for free is good. Zero-cost spells make an entire deck archetype known as Cheerios, and there isn't a single card that's more kill-on-sight than Omniscience. That's partly why the Leylines from Magic 2011 are so good: if it's in your opening hand of the game, you can begin with it already on the battlefield for free.
Leyline of Anticipation is a particularly powerful member of the Leyline cycle. With it out, you can play spells as if they had flash, which means you can play anything but lands when you could cast an instant. Creatures, sorceries, artifacts, other enchantments, everything's flash-able, allowing you to hold your resources open to cast whatever you like on other players' turns.
Other cards have similar effects, like Tidal Barracuda giving it to the entire table, but starting the game with a Leyline of Anticipation out is an incredible way to get way ahead of the game.
3 Propaganda
While you may associate taxing effects and 'Pillow Fort' with white, blue has a few fantastic tricks up its sleeve. In fact, blue did it before white, as Propaganda is just the blue predecessor to one of white's best enchantments, Ghostly Prison.
With Propaganda, your opponents can't attack you unless they pay two generic mana for each creature they control. In fact, it could be argued that Propaganda serves a bigger role for blue than Ghostly Prison does for white, as blue decks tend to have fewer creatures on the battlefield it could otherwise block with.
As most players have the bad habit of casting everything on their first main phase, it's incredibly likely they'll move to combat and realise they've splurged all their mana and can no longer afford to attack you. Some may see that as a challenge and gun for you even harder later on, but many also instead just redirect their efforts at enemies they can still cast spells before attacking.
2 Mystic Remora
Mystic Remora is easily one of the best turn-one plays you can make with a blue Commander deck. Costing just one mana, it forces your opponents to pay an additional four generic mana whenever they cast a non-creature spell. If they don't, you draw a card. In the early stages of the game, players will prioritise building their own board states and ramping their mana bases over dealing with you, so you're incredibly likely to get a few extra draws.
The only catch with Mystic Remora is its cumulative upkeep cost. You'll need to pay one mana for each turn it's been out on each of your upkeeps, or else you sacrifice it. Mystic Remora presents a trade off: do you want the cards, or the mana to cast them? Fortunately, you're likely to have gotten a lot of triggers off of it in just a few turn cycles, so you can comfortably sacrifice it before it gets too expensive to keep out.
1 Rhystic Study
The number one most popular enchantment for blue in Commander… is basically another Mystic Remora. Except it's also better in almost every way.
Rhystic Study might cost more than a Mystic Remora, it's still a more-than-reasonable two generic and one blue, and it doesn't have that pesky cumulative upkeep. For that extra cost, you could be drawing a card off of any spell your opponents cast, not just the non-creature ones – provided they don't pay one generic mana each time.
Still cheap enough to play in those early, resource-building stages of the game, Rhystic Study is one of those cards whose presence is always felt in the game for as long as it's out. It's similar to white's Smothering Tithe in that it has a pronounced psychological effect on the table. Every spell someone casts must be assessed on whether it's really worth one more mana or potentially giving you a card you could win the game with, and that's a lot of cognitive load to put on a player already trying to plan their turn.
The best thing about Rhystic Study is how deceptively small it feels. One mana isn't a lot, but neither is one card. Your opponents will often band together to agree to pay the one, but that kind of unity never lasts long. One opponent decides not to pay the one and tries to justify it with "well, it's only one card…", and the pact is broken. Nobody pays. You draw half your deck as everyone stops caring about the little Rhystic Study, and then you win with the 50 cards in hand your opponents have so graciously given you.