Using the Drupal 7's Block "description" field as a CSS class field
If I understood your question correctly, you have two options:

A) Use a _preprocess_block() function:

function THEME_preprocess_block(&amp;$vars) {
  // Make sure that this is a custom block, and not a block that is provided by
  // a module.
  if ($vars['block']-&gt;module == 'block') {
    // Load the block information.
    $info = block_custom_block_get($vars['block']-&gt;delta);
    // Add block description to the list of classes.
    $vars['classes_array'][] = $info['info'];
  }
}


B) Use the Skinr module. Probably will require more work, but it's much more flexible and editors / non-tech staff will be able to define styles of blocks. From the project page:


  Skinr's main purpose is to allow the theme to define a set of reusable and modular CSS styles, and to make those styles available in Drupal's UI.