Short answer: You can’t (at least not like this).
$page->blueprint() reads the blueprint and returns the values defined there. You cannot, however, change these values unless you would actually parse the blueprint, make your changes and save the changed values back to the blueprint file. But then you would end up with the same result, i.e. the same label for all pages that use the blueprint.
Having said that, you can use query language to change the label based on some condition via a custom method as explained here: Allow users to change field label and calling field label in template - #5 by texnixe (but instead of querying some field, you would use a method that returns the desired value).