Access current node in an existing block
Enable debug
by going in sites/default/ and copying the file default.services.yml to services.yml  and modifying the following lines
cache: false
debug: true

Turn browser comments on
make sure your browser has Comments enabled (some browsers have them disabled by default for example in chrome in inspector on the 3 dot menu click settings and preferences find Show HTML comments
Find out which twig template is used
after that inspect the code in near the block that you want to override and you should see something similar to this
&lt;!-- THEME DEBUG --&gt;
&lt;!-- THEME HOOK: 'block' --&gt;
&lt;!-- FILE NAME SUGGESTIONS:
   * block--block-name.html.twig
   * block--block-content--b436f9c6-34ae-4879-8b55-068015a999d3.html.twig
   * block--block-content--bundle-name.html.twig
   * block--block-content.html.twig
   * block--block-content.html.twig
   x block.html.twig
--&gt;
&lt;!-- BEGIN OUTPUT from 'themes/bootstrap/templates/block/block--system.html.twig' --&gt;   

you can get the twig block template from the location in comment BEGIN OUTPUT
Use and modify template
Copy this twig file into your theme templates folder and rename to any name above.  Modify the code and add something similar to the twig block template you just created
{% if nid %}
    nid:{{nid}} &lt;br/&gt;
    title:{{node_title}} &lt;br/&gt;
    body:{{body}}&lt;br/&gt;
    text:{{field_text}} &lt;br/&gt;    
{% endif %}

Expose node variable to block templates
Add into your THEMENAME.theme the following code or similar
function THEMENAME_preprocess_block(&amp;$vars){
    $node = \Drupal::routeMatch()-&gt;getParameter('node');
    if ($node instanceof \Drupal\node\NodeInterface) {
      // You can get nid and anything else you need from the node object.
      $vars[&quot;nid&quot;] = $node-&gt;id();
      $vars[&quot;node_title&quot;] = $node-&gt;getTitle();
      $vars[&quot;body&quot;] = $node-&gt;body-&gt;value;
      $vars[&quot;field_text&quot;] = $node-&gt;field_text-&gt;value;

      //following did not work ?!
      //$vars[&quot;field_text&quot;] = $node-&gt;get('field_text')-&gt;getValue();
      //$vars[&quot;field_text&quot;] = $node-&gt;field_text-&gt;getValue();
    }    
}

Clear cache
with the above you are making the node fields available to block templates