This page gives a very good overview of drupal 8 naming conventions, also for custom blocks:
Drupal 8 - Template naming conventions
This page says about block templates:
For example, "block--block--1.html.twig" would be used for the first
user-submitted block added from the block administration screen since
it was created by the block module with the id of 1. Region-specific
block templates are not available in Drupal 8.
If you had a block created by a custom module called "custom" and a
delta of "my-block", the theme hook suggestion would be called
Also one more example with Views, if you have a block created by views
with a view name "front_news" and display id "block_1" then the theme
hook suggestion would be:
block--views-block--front-news-block-1.html.twig (notice, when you
have underscores in a display id or in a view name - you have to
transform them in to a single dash)
Be aware that module names are case sensitive in this context. For
instance if your module is called 'MyModule', the most general theme
hook suggestion for this module would be "block--MyModule.html.twig."
Hope that helps. Have a nice day.