My sister is a harpist, and I know that among harpists tincture of benzoin
is used on the fingers to toughen them up and prevent blisters, (as when
for some reason there has been a lapse in practicing.  Harpists normally
have callouses on their fingers, and if they go for a time without playing,
they lose these and would develop blisters when they resume playing).
Tincture of benzoin is brown and sticky, and it has an odor as well.  It
has been used in surgical situations to make adhesive tape stick better to
the skin, in order to hold dressings in place over a wound.  It would
create problems, I would guess, for a breastfeeding baby (odor, taste,
stickiness, perhaps dangerous to ingest) and also is rather a problem to
wash off.  I don't know how all this would feel on a sore nipple, either.

Anne Altshuler, RN, MS, IBCLC and LLL Leader in Madison, WI