"Blister agents" refers to obsolete medications such as the extract from Spanish fly [0], which was used to try to help President George Washington recover from his cold:
"other treatments they gave [George Washington] during that period were enemas and drugs to make him vomit and something called blisters, where they applied Spanish fly onto his throat, which raises a painful blister, again to remove these terrible humors that are caution the inflammation."
-Bloodletting and blisters: Solving the medical mystery of George Washington’s death, https://www.pbs.org/newshour/show/bloodletting-blisters-solv...