Is it possible to backup created block as a module?
Exporting and importing blocks in Drupal 8 are quite easy (thanks to new configuration system). You don't need to create a module for this. In Drupal 8 configuration is stored in yml files. You can export and import it on the following page:

Administration >> Configuration >> Development >> Synchronize >> (import / export) >> Single item

It works for all kind of blocks except "Custom blocks" because they are actually not a configuration but content like nodes, comments etc. As a workaround you can use Simple block module mentioned above.

Note, that in some cases you also need to export block configuration dependency in a separate yml file. For instance for a block that shows five recent content titles you need to export underlying view and all its dependencies. You may also checkout Features module for some advanced configuration package tools.