Trying to come up with a poison that could be applied during a massage by a person wearing gloves. Ideally the effects are mild and slow acting enough it takes a number of sessions to kill the person. Ideally not too suspicious, begging for an autopsy. Could also be that the victim has a pre-existing medical condition (heart/kidney?) that is tipped over the edge with the addition of some chemical? My preliminary research turned up: arsenic (a bit too obvious), cyanide (ditto), thallium (promising!), Oleander (seems too old-time for my setting), ethylene glycol (maybe, tho the link to anti-freeze is kind of narratively blah). Since many people who receive deep tissue massages experience mild nausea and other mild ill effects, the immediate side effects could plausibly include these.
UPDATE: The dimethylmercury is appealing: I like the pace of it as well as the obscurity. Did some research, and it does seem like safety precautions - thick gloves, masking (so as not to kill my protagonist...) might draw some attention, so contemplating: having the masseuse suggest an acupuncture treatment to her patient. This would allow for needles dipped in the mercury, avoiding direct handling of the mercury. There's also a technique called moxibustion, which involves the burning of mugwort clumps on the protruding ends of the needle after it's inserted, so possibly our masseuse could claim a smoke allergy, necessitating the temporary use of a mask?