Programmatically change block content on page render only
You should probably try

$variables['content']['body'][0]['#text']= "&lt;strong&gt;html content&lt;/strong&gt;";


complete example 

function THEMENAME_preprocess_block(&amp;$variables) {
  if ( $variables['attributes']['id'] == "block-newblock" ) {
    $variables['content']['body'][0]['#text']= "&lt;strong&gt;html content&lt;/strong&gt;";
    $variables['content']['body'][0]['#format']= "full_html";
  }  
}