I think I've figured it out. A shortcut.
I've made a shortcut that can change contact photos. It's only semi-automatic. When you tap the shortcut, a list of your contacts appears, and you just tap the contact where you want to change the background, and the background is changed. So you don't have to go into each contact and select edit, add photo, find photo, apply, save.
I tested it on my iPhone on iOS 26.1, and it works.
Please note! This shortcut changes your contact's "avatar" or "profile picture" - and not the contact poster. However, your contact's profile picture will still reflect the color of the background, so if you follow my guide, you will get a completely black background, as in my screenshot further up. The disadvantage is that if you already have a picture on a contact, this will be overwritten with a black background.
Follow the steps here:
1. Create a photo album called Contact Photo in the Photo app
2. Download a completely black background, and save it to the Contact Photo album. Maybe this: LINK However, you can choose your own background. It is only important that there is only one image in the album.
3. Get my shortcut here: DOWNLOAD
You may be asked for permission to allow the shortcut to use your contacts. Grant it permission. Now you just tap the shortcut and the contact you choose will have a completely black background, as well as a completely black profile picture.
This is not quite what you asked for, but hopefully it can be a little help