JohnW, I work now in a laboratory for processing black & white movie film “National University of Theatre and Cinematography Arts “IL Caragiale” from Romania (http://www.unatc.ro/ ).
Since 1980, when I work in film, preparation of solutions for film development from raw chemical with a purity were photo standard .
Purity of these substances was around 97- 99 %, according to the standards for photo use.
High purity chemicals we use in laboratory chemical analysis (reagent).
I assume that your`s boric acid can be used without problems in the preparation of the developer.
Borax with boric acid is pH buffer.