During the Drupal 8 port of my relatively new Mefibs module I realized that block creation in code got somewhat more complex than it used to be in Drupal 7 and earlier. It took me quite a while to figure out how this works, especially because there is not a lot of documentation for Drupal 8 yet. So I had to dig my way through core to see how it's done there and finally I got around it. The following is a comparison between how we used to do this in Drupal 7 and how it's done in Drupal 8.