The Boston Library Consortium (BLC) welcomed The Boston Public Library (BPL) back into the consortium on July 19, 2022. The BPL policy for issuing a consortium card is:
Undergraduate students as well as employees of the Commonwealth of Massachusetts holding a library card for the State Library of Massachusetts who are unaffiliated with an academic institution may apply for a Consortium Card when they can demonstrate a specific scholarly research need.
You (a BPL card holder) would need to apply for a consortium card through Boston Public Library; where it will be determined whether a card can be granted. Once you have a consortium card, you will be able to check out books according to the policies for guest borrowers.