Forking is a common way to modify an open source project to make it your own. There are many successful forks of Blockly, including pxt-blockly, scratch-blocks, and App Inventor.
However, forking Blockly can make it difficult for you to pull in updates and bugfixes in core Blockly. We strongly recommend that you customize Blockly using plugins instead of by forking.
For more information on plugins and available APIs, see Plugins.
Ask on the forum
Someone else may have implemented the behaviour you want. Search the forum for previous discussions, or post and ask if anyone else has already written that code.
Use an existing plugin
If your change is a commonly requested feature, we may already have published it as a plugin on blockly-samples.
Write a plugin
Write code that uses Blockly's publicly available APIs to make the change you need. For instance, changes to block rendering, toolbox appearance, and connection checking behaviour can all be implemented as plugins.
File a feature request
If you need an API that isn't public, you can file a bug against core Blockly to make that API public.
Make a pull request
Blockly welcomes contributions! If your change is general-purpose, your best bet may be to make a pull request. Blockly improves, you don't have to maintain a fork, and everyone wins.
Check out the contributing page to learn more.
Updating a fork
We recommend that you merge in the latest version of Blockly on a regular basis. We publish Blockly quarterly, and each release includes release notes and a discussion of any breaking changes.