Source files: edx-documentation/en_us/xblock-tutorial
The Open edX XBlock Tutorial seeks to show users how to create an XBlock.
Mark Hoeber created the XBlock Tutorial in 2015. It has not been regularly maintained.
Per Google Analytics, the home page for the Open edX XBlock Tutorial gets roughly 35 visits per week.
Xavier Antoviaque of Open Craft has a separate XBlock tutorial, and some reports have said that the Open Craft tutorial is more complete.