What training is required for a class B endorsement?
I can not find any guidance other than what you have already noted for Class B airspace endorsements. The ground and flight training requirements seems to be left up to the discretion of the CFI. Though, it seems to be very similar to a solo endorsement as having criteria that is subjective to student and instructor.
Similar to a solo endorsement, a conservative CFI would only endorse the student for a specific airport, in a specific Class B airspace (and not the actual Class B airport), for a specific timeframe, and specific conditions. Even though the endorsement must be given within the preceding 90 days of the flight, the conservative CFI would specify a specific date and time instead of a blanket authority.
That being said, your entire logbook would then serve as a record of adequate training. Previous corresponding flights with the CFI would be logged in the logbook as evidence of training.