Blisters can be caused by straps that lay across the toes (as in my case) or from shoes that don't fit well. Shoes that are too big and cause irritation from the foot moving around in the shoe, or from parts of the shoe that are too tight. Your body tries to protect itself from the irritation by producing fluid as a cushion that collects under the skin.
The best way to protect a small blister while you need to continue wearing those same shoes is to apply something that conforms to the toe to prevent further friction.
I like to carry a self adhesive 1 inch bandage with me at all times. One name brand is Coban. You just wrap it around the affected toe.