The scope of the terms is easiest to compare side by side.
ObjectFaṣāḥahBalāghah
Single wordYesNo
UtteranceYesYes
SpeakerYesYes
At word level, sound combination and conventional usage are central. At utterance level, syntax and the intended sense also matter. In a speaker these qualities are stable abilities, classically termed malakah (ملکہ).