Print Taxonomy Term in block.tpl.php
Here is code tested to work on a minimal install with nodeblock. It's well commented but to recap: get the node displayed in the block, get the term, use one the term's field as css class.

// change mytheme to your theme's name
function mytheme_preprocess_block(&amp;$vars) {
  // don't do anything if it's not a nodeblock block
  if (empty($vars["elements"]["#node"]) || !is_object($vars["elements"]["#node"])) {
    return;
  }

  // get the node object, already loaded
  $node = $vars["elements"]["#node"];

  // get the linked term's id (change field_term to yours)
  $tid = $node-&gt;field_term[LANGUAGE_NONE][0]['tid'];

  // load the term fully to get access to its fields
  $term = taxonomy_term_load($tid);

  // turn one of the term's field into a css class (change name to any field)
  $css_class = drupal_clean_css_identifier($term-&gt;name);

  // add the css class to the list of classes for this block
  $vars["classes_array"][] = $css_class;
}