http://doctoryes.github.io/openedx_talk_xblocks/#1
http://edx.readthedocs.org/projects/xblock/en/latest/
If you haven't yet gone through the tutorial, do so!
http://edx.readthedocs.org/projects/xblock-tutorial/en/latest/
Braden MacDonald has also made a great presentation for OpenEdxCon 2016 on making an XBlock:
Adding a New XBlock on Production
Which XBlocks exist and are available for use? Where's the XBlock code? Check here: