Hi! A blueprint is a package or container for composing focus-specific sets of standards, patterns, and requirements related to the implementation of Azure cloud services, security, and design that can be reused to maintain consistency and compliance.
A policy is a default allow and explicit deny system focused on resource properties during deployment and for already existing resources. It supports cloud governance by validating that resources within a subscription adhere to requirements and standards.
Including a policy in a blueprint enables the creation of the right pattern or design during assignment of the blueprint. The policy inclusion makes sure that only approved or expected changes can be made to the environment to protect ongoing compliance to the intent of the blueprint.
A policy can be included as one of many artifacts in a blueprint definition. Blueprints also support using parameters with policies and initiatives.
Azure Security Center provides a higher-level management view of your policies.
This documentation does a really good job explaining the differences for your use case! Please let me know if I can clarify anything for you!