An API Blueprint developer designs, documents, and manages APIs (Application Programming Interfaces). They use the API Blueprint, a high-level language, to write API descriptions and documentation, facilitating clear communication about the API's purpose and functionality. Their responsibilities also include defining endpoints, requests, and responses, and specifying data structures. They often collaborate with other software developers to ensure the API aligns with application requirements. Additionally, they use tools like Apiary or Dredd for testing and validation to ensure the API functions correctly. They play a crucial role in creating efficient, reliable APIs that aid in software integration.