How do I print individuals fields in block.html.twig when is a node in a block?
You have to set "node" in the twig-variables of your block. I.e. you can do this in a template_preprocess hook like so:

function mytheme_preprocess_myBlock(&amp;$variables) {
  $variables['node'] = Node:load(50);
}


Then you can access the "node" in your template like {{node.my_field.value}}.

The hard part is to find which node to load in the preprocess function :-)