Why do people refer to Blue color as "U" in Magic the Gathering?
According to Magic Head Designer Mark Rosewater's "Drive to Work" podcast on the subject, Wizards used "B" for Black and "L" for Land, leaving "U" as the next reasonable choice.


  Blue we represent with the letter U. Oh, real quickly. Why do we do that? I’ve talked about this in my column, but for those that haven’t heard me say it, when first Richard made the cards, the problem was that black and blue both start with B. So why not go to L then? Well, land...card colors are used to signify the border, the kind of card it’s on. And so land has its own border. So L is for land. Well, why not go to A for black? Well, A is artifact. And so U was the first we could get to in either of them, so we ended up going with U.