Load external libraries in a custom block
Libraries don't work like that. You need a yourmodule.libraries.yml and define it there.

There's detailed documentation on https://www.drupal.org/developing/api/8/assets#external. The following is an example of how a library is defined.

angular.angularjs:
  remote: https://github.com/angular/angular.js
  version: 1.4.4
  license:
    name: MIT
    url: https://github.com/angular/angular.js/blob/master/LICENSE
    gpl-compatible: true
  js:
    https://ajax.googleapis.com/ajax/libs/angularjs/1.4.4/angular.min.js: { type: external, minified: true }


Then, the library that you specify is yourmodule/nameofthelibrary, see https://www.drupal.org/developing/api/8/assets#render-array or any of the other examples on that page.