THe fiddle doesn't show anything. Pretty pointless.
Can you use an image editing program to darken the image and serve that? That will avoid having to make an on-the-fly transformation on page loads. Also is the text static? If so perhaps having a static image with the text on it would be easier. You don't give an info with regard to your needs so difficult to suggest a solution.
You can apply text server side with php's gd commands.
You can over lay text very easily using css /js.
JS libraries can use darken as can css.