Service blueprints are divided into layers from the customer and physical evidence, to the front of house employees and systems, to the back of house employees and systems. Journey maps seem to be a bit looser, and usually involve the customer's emotions. What's the use case for using a service blueprint instead of a journey map?
Key here is Implementation vs. Discovery.
These models are used slightly differently depending on life-cycle
Designing a New UX
Service blueprints identify clearly what a company needs to get in-place to service defined customer needs (implementation of business processes)
A Customer Journey should start with understanding the ideal world of a customers wishes and desires. From that see what services could fit seamlessly into this customers world. (discovery of user experience)
An anti-pattern would be drawing a "Customer Journey" on top of existing company structure. Don't expect delightful UX if the expectation is that users should learn to work with the companies way of doing things.
From the above there clearly is a "horse and cart" relationship between the two. Discovery of ideal user journey should drive company structure. By way of example, according to this article Google re-organised to deliver 'Google Now' product
Understanding a Current UX
A Customer Journey would show complete picture of the activities to accomplish a high level goal, both where they interact with the organisation and their other tasks. Ratings of satisfaction with the process is often useful data (Discovery of customer pain-points)
Service blueprints identify how the company structure is organised and operates to service the customers needs (understanding Implementation of business processes). This organisation may influence the Customer Journey.