مَا جَاءَنِي زَيْدٌ بَلْ عَمْرٌو
It was not Zayd who came to me; rather, ʿAmr came.
The first phrase negates Zayd's arrival, while «bal» directs attention to ʿAmr as the affirmed arrival. This is not an additive statement that both came; it is a correction from a negated referent to the intended one.
Although the same particle occurs in positive and negative contexts, its relation to the preceding assertion is not identical in all respects.
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ʿAṭf al-Bayān, Badal and Coordination: Clarifying, Reinforcing and Specifying the Subject
مَا جَاءَنِي زَيْدٌ بَلْ عَمْرٌو
It was not Zayd who came to me; rather, ʿAmr came.
The first phrase negates Zayd's arrival, while «bal» directs attention to ʿAmr as the affirmed arrival. This is not an additive statement that both came; it is a correction from a negated referent to the intended one.
Although the same particle occurs in positive and negative contexts, its relation to the preceding assertion is not identical in all respects.