Music Lab by Code.org is a Blockly-based experience, built for K-12 education, at https://code.org/music https://code.org/music. It's open source, and we wrote

Music Lab by Code.org is a Blockly-based experience, built for K-12 education, at https://code.org/music https://code.org/music. It's open source, and we wrote some technical documentation at https://github.com/code-dot-org/code-dot-org/blob/600ebafa524bb0299b77662ddb5ed86906ff8b47/apps/src/music/README.md https://github.com/code-dot-org/code-dot-org/blob/600ebafa52.... There were a bunch of interesting aspects to this project. One of my favorite things was developing the user programming model. Organizing your music using functions is very powerful.