Clear Image Overlay without Clearing caption and making invisible

I know I’ve done this before, but am drawing a blank.
I have an av_image element with a caption. It’s a white caption and a very dark image.
In just this one instance, I want to make a clear overlay for non-hover and for hover — i.e., always!

I know how to do custom css for single id or class, but could you help me out with the css to make the overlay clear instead of with color?

I know how to go in the admin to image styling and clear the caption overlay background color, but can you tell me the css — I will potentially need to do on multiple images and find css just easier.

Thank you for any assistance.