I'm trying to display a rich text component containing a warning message on Account. I'd like to display it the Accounts related Contacts have a blank email. Is there any way to do that? Any help is greatly appreciated.
Using standard set component visibility, You can not achieve that. You can set visibility based on User, permission or record, if you select record you will not get the Contact field to use for setting the visibility.
You can use parent information of record only, not the child for setting the visibility using standard set visibility functionality.
Create a Aura Component or Lightning Web Component with same functionality and add it to Account page in Lightning using Lightning app builder.
As you can only use fields from the record being viewed, you'd have to have some field on the Account that would flag that at least one child has a blank email address.
I'd likely create this field as a simple checkbox, and build trigger/ process builder/ workflow on Contact that marks the checkbox on the Contact's Account as true if the Contact's Email field is blank.