Kismet:
If the blister is painless, it comes most likely from unremebered trauma. Even people a lot younger than 88 often don't recall the minor bump or
scrape which produced the alarming purple
bruiseBone bruise
Bruise
Bruise healing - series
Muscle bruise
Skin bruise or blister they see one day.
I think, however, that in light of her advanced age, your mother-in-law would be better off calling her doctor for advise. Puncturing the blister is, as you suggest, not a good idea. If it's just a blister, time will take care of it with just padding and time (perhaps a week.)
Best.
Dr. Rockoff