Note: THIS IS VERY OLD CODE
A slightly newer tool for helping get started with plugins: https://www.npmjs.com/package/generator-blueocean-usain you can also see a real plugin: github.com/jenkinsci/blueocean-pipeline-editor-plugin
Blue Ocean Extension Point implementation sample plugin.
Please check out the following YouTube video for a walk-through of the concepts relating to client-side Extension Points in Blue Ocean, as well as a walk-through of this sample plugin.
And here are the slides.