blockUI : block() function does not seem to respect local options overrides

blockUI : block() function does not seem to respect local options overrides

According to the blockUI documentation, options can be overriden either globally or locally. Thus, to override the message class globally you could do: 
  1. $.blockUI.defaults.blockMsgClass = 'progress-wrapper';

..and to override the message class locally you could do:
  1.  $('#targetDiv').block({blockMsgClass: 'progress-wrapper'});

However, after fiddling back and forth with this, I'm unable to override locally. As far as I understand, the following piece of code should work:
  1.       jQuery("div.search-results").block({
  2.         blockMsgClass: 'progress-wrapper',
  3.         message : '<div id="search-results-loader"></div>',
  4.         centerY: false,
  5.         css : {}
  6.       });
The above should (1) apply the class 'progress-wrapper' to the message div (instead of blockMsg) and (2) use an external stylesheet as the 'css' option is an empty array. However it does not work.

If I do a global override, things do work.
  1.       jQuery.blockUI.defaults.css = {};
  2.       jQuery.blockUI.defaults.blockMsgClass = 'blocking-message';

  4.       jQuery("div.search-results").block({
  5.         message : '<div id="search-results-loader"></div>',
  6.         centerY: false,
  7.       });
Is this (1) behaviour by design, (2) caused by something I do wrong or (3) something different?