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That behavior is correct: null doesn’t mean anything at the database level when used with a ManyToManyField. The declaration of a ManyToManyField causes the creation of an intermediate table to hold the relationship, and although Django will create a standard attribute on your model instance for your convenience, there is no actual column representing it that could be nulled. By definition there can always be zero instances of the relationship.
blank=False, though, does have an effect on validation (when using model forms like the admin app, for example), forcing the user to choose at least one relation.
(Note that Django’s built-in migration system creates migrations for just about any change to a model, regardless of whether it affects the database or not. So this change could lead to a migration, but it wouldn’t affect the database or whether or not you could have zero instances of the relationship.)