What problem does this address?

It would be great to have the option to set a default duotone filter for the blocks from within the theme. That would allow theme authors to fit the images/media on the site to fit with the color scheme of the theme.

What is your proposed solution?

Something like this on theme.json would be nice to have:

"styles": {
    "blocks": {
        "core/image": {
            "color": {
                "duotone": "my-duotone-slug"
            }
        }
    }
}

@ajlende