How can I add fields to a block?
You can create a Custom block type (admin/structure/block/block-content/types/add) and then add fields to it (admin/structure/block/block-content/manage/YOUR_BLOCK_MACHINE_NAME/fields). 

You will see that you have the Manage form display and Manage display tabs too like the content types, you don't need to create a content type. In Drupal 8 you can use blocks in more than one region.

Check The Block System is Finally Useful in Drupal 8 and see the video.