Well, you can always target by touch, even when blind...
Also, one of my players used a crafty solution to the problem during one major battle awhile ago. They were fighting green slaadi with sorcerer levels and magical staffs, and each staff had a unique appearance about it. When one of the slaadi cast Blind on the party cleric, the cleric used Locate Object to determine the location of that slaad's staff, and then used that information to target a Spiritual Weapon spell down upon it's beany little head...