I took my SBB a hundred years ago (OK, maybe early on when it was first computerized).
I self-studied. Did not go to SBB school, did not take an online class. I had, and still have, the great fortune of working at a world class hospital.
What worked for me:
Read Transfusion cover to cover for at least a year. Regardless if I was interested in the article or understood it.
Read the AABB Technical Manual, at least once, I think I read it twice, maybe three times.
Memorize all of the common antigen frequencies, and some of the odd ones too.
Read where the concentration of questions would be. For example, there was anticipated to be 1 HLA question on my exam. A person could get their PhD in HLA, I only read that chapter once.
Memorize AABB Standards.
Memorize the relevant FDA CFRs.
Read a good hematology text book.
That is a lot of reading and will have you well versed in most things related to blood banking. You will easily pass your BB.
If you are taking your SBB, the only thing I was not anticipating was the management questions. I did not study anything about that. If I were to take it again (that will never happen), I'd read a book or two on employee relations / management.
I was one of the lucky few who passed (barely) the first time. But you know what they call the person who finishes last in their class of medical school...