Fasāḥah asks whether the utterance is linguistically sound and clear. Balāghah asks an additional question: why this particular form of words suits this particular situation.
A specialist asking about a familiar term may need a concise clarification rather than a lengthy elementary lesson. A beginner may need fuller explanation of the same term. Both responses could be linguistically sound, but their contextual fitness differs.
Consequently, a balīgh utterance must be faṣīḥ, yet not every faṣīḥ utterance is automatically balīgh in every situation.