1) The monsters can look like anything you like. Even if you take an existing monster, you can refluff it however you want.
2) There are rules in Pathfinder for increasing the CR for monsters. Here are some
Templates
3) If you find that your players are too strong for the monsters spells, you can trade it for a higher level, similar spell somewhere else. Alternitively, you can scale the spell up. A lot of Blue Mage spells don't scale automatically (or at least, they didn't, haven't checked in a while). Add a few d6, maybe an extra effect. Keep in mind, if you give a monster a spell from another list, or an improved version of a blue mage spell, the player might get access to it (Blue Mage)
4) Add HD, stats, maybe increase size category.
5) Take existing Pathfinder monsters and add blue mage spells to them. This should either bump the CR up a bit (depending on the level of the spell) or replace an existing ability. You might also consider giving them non blue mage spells, but again, Blue Mage.
6) Don't go overboard.