Faerie Fire, True Strike - both 1st level spells
Area effect spells, chained/twinned spells (hit both targets)
Hitting the group with Dispel Magic - not only works vs blur, but can counter other spells as well as they get access to them -- and avoids looking like your targetting the one guy for one spell.
Or you can go the other way, and conjure up fog or darkness and give your own group 50% chance of being missed.