I agree with you both.

Technically the source is a blueprint. But this comes down to scale.

Among other things I've worked on banking and corporate management applications. You cannot reason about the codebase at that scale. In these instances it's not possible to have a good outcome without a blueprint.

Using the building metaphor, you can get away with building a bird feeder without a blueprint. You can't get away with building a skyscraper without one. Houses, maybe. Either way, high level blueprints help everyone.