Unable to show the block title in D8
You are supposed to put your templates into a /templates subfolder of your theme.

I'd recommend you find out how to enable debugging for Twig templates during development. Here is a how-to on DO: https://www.drupal.org/docs/8/theming/twig/debugging-twig-templates

By that you'll get template suggestions printed as HTML comments directly inside the markup of your page. It will suggest you certain file namings which then must be placed inside the /templates subdirectory of your theme.

And last but not least: Flush caches as often as possible. And find out how to disable caches completely during development. Here is another how-to on DO: https://www.drupal.org/node/2598914