To darken a background image using CSS, you can employ a pseudo-element. Wrap your image in a div and add another div with a semi-transparent overlay. Use the following CSS: `.overlay { position: absolute; top: 0; left: 0; width: 100%; height: 100%; background: rgba(0, 0, 0, 0.5); }`. This will achieve a darkened effect over your background image.
