The emphasis on the period here is not significant. 9.1.2 of the UEB code says: Typeform indicators are considered necessary in braille when the print change in typeform is significant because it indicates emphasis or shows distinction. 9.7.3 of the UEB guidelines says: ignore a change in typeform for incidental punctuation within a passage.
The answer here is...you choose and be consistent. There are arguments that would say that a word indicator should be used as the period is clearly bold in print. There are arguments for the idea that the punctuation is for the end of the sentence - it does NOT apply to the letter B - that makes it insignificant to the bold phrase and, as such, a symbol indicator should be used. It won't significantly change the way it reads either way.
Cindi