Currently, if you add a Cover block, the overlay opacity option only appears if you add a background image. If you use a solid color or gradient only, you get no opacity control. Of course, the original intention of the opacity option was for letting the user choose how much of the image bleeds through. So, it makes sense that it doesn't show up.

However, there are use cases where users might want to have a gradient/color that is transparent, particularly when a Cover block is nested within another block (like another Cover).

One of the patterns I've been working on uses a Cover nested inside of a Cover. The inside Cover uses a transparent color overlay now (with a framed border block style). It looks like this:

If I add a gradient color (no image) to the inner Cover, it looks like the following (no bleed through of the outer Cover):

What I want to achieve is the following, which would require control over the opacity:

It is possible to achieve this using rgba() for gradient presets, but we're talking adding 9 extra gradients for each existing gradient (to handle 0.1 - 0.9 transparency). And, that, of course, would be insane. :)