A few things I've learned playing Blue Multiplayer:
1: Like it was mentioned above, stealing creatures really works well. Sometimes you can steal creatures with weird abilities and do crazy things, like stealing a cleric and healing other peoples stuff, or stealing an Ernham Djinn and giving the 5/5 Black creature Forestwalk.
2: Big Fat Flyers. Don't go overboard and make everyone mad, but having piles of Big Flyers is a blue specialty. Go with it. Especially good are the 2/4s like Ghost Ship and Spire Golem. They don't have high power and aren't too scary, but at the same time block real nice.
3: Counterspells are not for control, they are for emergencies. I usually pack 4-12 counterspells in a Blue Multiplayer deck. The key is to know their role. You are not going to control anything with counterspells in a multiplayer game. What you can do however is save them for when somebody casts something that will really mess things up. Just the threat of them can usually change other peoples games.
4: Most important of all!!!!!!! BE DIPLOMATIC!! You're playing blue and you have a target on your chest. You need to keep everyone from getting angry at you. If you start putting out a pile of flyers, don't attack with them. Attack with one of them, against somebody who can take it. Do as much damage with flying as you think you can get away with, and nothing more. Counterspells can also be social. Especially if your playing a 4 player game, and when you counter something, the other 2 players thank you. Don't try to control things, or mess up anyones game. Just pop off the minimum countermagic to prevent anyone from gaining the advantage. Make alliances. If somebody targets you, pick somebody else and offer them friendship. "Help me deal with him, and I'll leave you alone"
5: Lay in wait. If your opponents decide to gang up on you because you're blue, there's really nothing you can do. If they are polite enough not to do that, you now have a goal. Sit back, be inconspicuous, and aim not at winning, but at staying alive and being one of the last 2 players. Blue can't beat 4 people but it sure can beat 1. Stay out of the way, and try to get everyone to kill each other. Try to keep a low profile, let them beat on you, and kill your stuff as long as your survival isn't threatened, but keep yourself strong enough, and have enough held back that when its down to 2, you can gain control. If someone is almost dead, help finish them off. Your goal is to get others killed off.
Sure this stuff doesn't work if people decide to take you out, but I've found in simple friendly casual games, people generally don't do that. My friends might be weird, but that's how I've found it. Its amazing how well I've done over the years playing simple Big Blue decks or W/U Swords/Serra/Wrath/Counter/Control Magic in multiplayer games. It just takes some finess, and knowing what you can and cannot do.
