The current generation of block languages, with its focus on teaching programming to novices, has not been designed for professionals. In this paper, we argue that blocks-based languages aimed at professional end-user developers face requirements that present challenges to the user interface design of such languages. We discuss three aspects that set potential professional use of block interfaces apart from educational use with children and students, and their implications for the design of blocks-based language editors. These aspects are that professionals: (1) require the editor to support high-productivity, (2) should not be limited by a simplified run-time environment, and (3) need the blocks editor to provide support for working with large programs. These three aspects provide research avenues for extending the usefulness of blocks-based language interfaces. We intend to explore these aspects with the design and development, of a blocks-based prototyping system for web designers. We report some preliminary results from an initial user experience-study in which 4th-year web design students were exposed to a blocks-based version of a language they already knew.