Though the usage has been criticized, not only ... [but] also functions as a coordinating conjunction even without the contrasting but, thus not requiring a semicolon to join two independent clauses. Since there appears to be a question whether this is indeed the case, look at the clauses without the conjunction:
Walker was a trailblazer in the medical field.
She was a visionary who bravely challenged many of the social prejudices of her day.
Both clauses have the structure Subj. + copula + predicate noun — clearly independent clauses.
The lack of the contrasting but does, however, suggest a colon might even add more drama to this sentence, as others have suggested. Whichever you decide, a semicolon would be the wrong punctuation here.