I've found this code below:
  if (opt_options) {
    // TODO(scr): don't mix this in to global variables.
    goog.mixin(Blockly, Blockly.parseOptions_(opt_options));
  }

If I put callback to options it would be added to Blockly object as property. I suppose we don't need to store this callback for all life cycle. Also we could get property name collision and replace some valuable value to this useless callback.