Debbie
I recently saw a large milk blister (bleb) that was a bit like you
describe.  It came up in a lump because it was blocking the openings
to some milk ducts and when it was opened it had a serous fluid, a
bit milky.  She had blocked duct symptoms above that area.  It was
particularly tenacious - lancing it with a sterile needle dod not
help.  Eventually her doctor had to cut the whole thing off and then
she used USP modified lanolin for moist wound healing and to stop any
scab forming to block the area again.

Ros Escott BAppSc IBCLC
Tasmania, Australia