1. For GB Container Blocks, you can add a Background color, and set a lower value for the opacity to set this. It’s not ideal to do this globally through custom code I believe. It can only work if you’re sure that no Container Block will be using the :before pseudo element.
If you’re referring to backgrounds set through the customizer, we can do it through custom CSS – targeting each element one by one.
1. Activate the Backgrounds module in Appearance > GeneratePress
2. Customizer > Backgrounds and set the Body background image.
3. Add this to make it blurred:
body {
backdrop-filter: blur(20px);
-webkit-backdrop-filter: blur(20px);
}