Calling nitrogen mustard a ‘blister agent’ is unhelpful in this context.
"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...