Bro nycazn is correct.
BM will mean the girl will be taking her getting her tits out for your pleasure at a minimum, most likely draping them/rubbing them on your back/chest whatever, and allowing you access to paw them/suck on them.
Some girls will strip totally naked for their version of BM, and it's a similar story, titty rubs, access etc. Other girls when giving BM will give you a nice oily tit wank, so you can shoot your load that way. In my experience it's rare that a BM means BJ.
If you opt for Massage+HJ only, you may find that, again depending on the girl, your degree of access ranges from less to significantly less - she may not disrobe at all, so no tits for you.
Every girl is different and will offer a slightly different service, but if want a general guarantee of seeing tits and maybe/probably nakedness, opt for BM.
