A Ritual Blueprint is the complete mini-game or icebreaker that you can play with another user after a successful Handshake. It's the full template for an interaction. For example, the "Two-Note Name Game" is a complete blueprint.
A Ritual Challenge is one specific part of that game. It's the individual action you take. In the "Two-Note Name Game," a challenge might be to remember your partner's two-letter name snippet. You complete several challenges to finish the blueprint and earn Spark Credits.
To help clarify the idea, a "blueprint" is a predefined plan or set of rules and a "challenge" is one of the rules.