Making blog banner background black with opacity

Hey guys! Does anyone have a work around for making the background of the banners images opaque with black background, I have tried:

background: black;

opacity:50%;

but these don't see to work, any help would be amaze!

(this code is supplied via Will Myers in a free tutorial, the code is added on the page and it pulls the colour styling from the page, but I dont really want to have an entirely black page, just so my banner looks ok)

Your code missed a selector (tag, class name or id)

You can share link to page where you use banner? We can help easier