Mrs Blather noticed a single watery blister on her arm just smaller than a dime. It looks like what you would get if you splashed hot grease on yourself, but she was not cooking. It is not in an area that rubs against anything. Can an insect bite cause this? I usually equate that with more of a raised bump.
Any recent injuries? It could be a cyst.
Maybe it’s a wart, but they tend to have a more solid feel. (IANAD, and all that rot…)
Any chance she rubbed against some poisonous plant? Because I once had a row of three water-filled blisters pop up on an arm after I casually brush some ‘ivy’ aside during a walk.
Poison Ivy/Oak would be possible, but there is just a single blister.