Darken background image on hover

How about this, using an overlay?

.image:hover > .overlay {
    width:100%;
    height:100%;
    position:absolute;
    background-color:#000;
    opacity:0.5;
    border-radius:30px;
}

Demo

Similar, but again a little bit different.

Make the image 100% opacity so it is clear. And then on img hover reduce it to the opacity you want. In this example, I have also added easing for a nice transition.

img {
    -webkit-filter: brightness(100%);
}

img:hover {
    -webkit-filter: brightness(70%);
    -webkit-transition: all 1s ease;
    -moz-transition: all 1s ease;
    -o-transition: all 1s ease;
    -ms-transition: all 1s ease;
    transition: all 1s ease;
}

That will do it, Hope that helps.

Thank you Robert Byers for your jsfiddle