I think you are mixing two different concepts.
Balsamiq is about creating wireframes. Wireframes are there especially to not mimic too much the "final" product so that you can concentrate more on the feature an flow aspect without suffering of the interference that the visual elements could create.
Keynote, at least the way you are using it, is to create mockups. Mockups are there to crystalize (or invalidate) the ideas expressed in the wireframes together with the branding you are creating for your application.