Create a new map in Kumu, open up the javascript console, and point the map at the blueprint:
Workflows.setCurrentMapSource("https://example.com/path/to/blueprint.json");
For example, if you wanted to load the blueprint below you would use
Workflows.setCurrentMapSource("https://gist.githubusercontent.com/rymohr/06533f749645f13c7732/raw/420f21f74b6de68485347af058f1a3f8b12b89a9/simple-kumu-blueprint.json");
The map data will now be fetched on the fly each time you visit the map. Just make sure to validate the json before trying to load it in Kumu. The error messages you'll get within Kumu won't be helpful here.
See https://kumu.io/rymohr/remote#simple for the final result!