Fasaha concerns the quality of wording and construction; balagha additionally tests whether the utterance is appropriate to a given rhetorical circumstance.
SituationAssessment
Fasih and contextually fittingCan count as baligh
Fasih but inappropriate to the situationNot baligh in that setting
Context considered but language unfasihDoes not meet the full definition of balagha
Thus fasaha is a prerequisite of baligh discourse, but a fasih phrase is not necessarily baligh in every possible setting.