My two cents
The way you create a service blueprint depends on what you are using it. Are you using the service blueprint to describe the actual state of a service, or are you using it to imagine a future state of a service?
Steps to describe the actual state of a service with a service blueprint:
Research: do your research to understand how people really use the service and what employees do to make it happen (for example user and stakeholder interviews, observation, surveys, gathering quantitative data)
Build a service blueprint: build a service blueprint that summarises how the service works with multiple stakeholders that share their expertise and use the previous research to ground the blueprint in data.
Improve the service blueprint: present the blueprint to other stakeholders who weren’t involved yet to make sure all parts really fit the reality and improve it based on feedback.
Make it shareable: make the service blueprint shareable within the company so that different work groups and departments can use it.
Steps to imagine the future state of a service with a service blueprint:
Ideation: gather ideas about how the future or new service could work.
Build multiple service blueprints: summarise the best ideas in multiple service blueprints that show different approaches
Test the idea: use the service blueprint to test the service or guide the prototyping phase.
Condense: based on the testing, choose the winning service blueprint and improve it.