Animate the backgroundColor-opacity-value.

backgroundColor-alpha-blend.js

$.fx.step.backgroundColor = function(fx) {

  if (!fx._init) {

    // parse RGBA-Color
    var col = /\((\d+,\s*\d+,\s*\d+),\s*([\d\.]+)\s*\)/.exec($.css(fx.elem, 'backgroundColor'));
    
    // set correct start-value
    fx.start = parseFloat(col[2]);
    
    // remember background-color
    fx._baseColor = col[1];
    
    // run only once
    fx._init = true;
      
  }
  
  // do the job
  fx.elem.style.backgroundColor = 'rgba(' + fx._baseColor + ', ' + fx.now + ')';
}