Showcase slider type - Dark overlay

You can create the a dark overlay effect by following the steps below.

Switch to Expert mode before proceeding.

Adding the background color
Add a background color to the Slider settings → General tab → under Showcase slider type - Settings: Slider → CSS. E.g.
background-color: #181e24;	
If you're running 3.3.12 or newer you can also use the Slider background color.
Add an opacity to the slides

Scroll further down the page and find the Opacity setting under Showcase slider type - Animation. Turn it on and adjust the Before and After values.