How to create a block item In Drupal 8 Using RESTful Web Services ?
Before you create Rest API you must have enable core Web services module list & Install custom "Rest Ui" module and enable it.
Here are the steps to create a block item
Step:1 Go to Admin/Configuration/Web-services/Rest and enable "Custom block"
Step:2 select all checkbox and save.
Step:3 Go to Admin/structures/views/add new view
Step:4 Follow below steps of view
Step:5 Select in view contextual filter/add
Step:6 Now you can view block items