Please advise how to sanitize my acrylic retainer after a fever blister?

Bleach and water: The best way to sanitize an oral appliance is to soak it in half bleach and half water for five minutes a day during the breakout. Do not soak it longer than that, and do not use a stronger concentration of bleach. Then rinse and brush it well to get rid of the bleach taste.

Cleaning retainer: I would brush and rinse your retainer the same way you clean your teeth, with toothpaste and cool water. Some families rely on hydrogen peroxide rinses, like peroxyl, to decrease the incidence of these outbreaks for family members. Discuss any irritations with your orthodontist and practice good oral hygiene as a good first step to solving this problem.