Well, 'Ben Hur' blurs the line because it takes historical events and settings and weaves fictional characters and their narratives into them. For example, the chariot races were a real form of entertainment in the Roman era. But the specific race involving Ben Hur and his rivals is a fictional account. Also, the religious aspects in the story are fictional interpretations within the historical religious context of the time.
It blurs the line as it uses historical context to ground its fictional story. The Roman occupation and its associated politics are historical facts. But the story of Ben Hur's family's downfall and his rise against the Romans is fictional. The novel uses the real - world framework of the Roman Empire to make the fictional story seem more believable and to give it a sense of authenticity that it wouldn't have if it was set in a completely made - up world.