First, search for "box" in the left menu and drag & drop a solid-color box to the editing space. In this example, the box's color doesn't matter as we can change it later on.

Click on the box to resize and change the color to black. Then, click on the arrow at the end of the bar to change its transparency.

I'll set the transparency at 28. Now, click "back" to move the transparent layer backward.

Quick tip: The transparent layer is extremely useful for complicated design as it shifts the focus back to the main content.