Background vid elements are a bit different- you cannot bind them to a CMS field which means there’s no way to vary that content by page.
If you only have one on all of your CMS pages, you could just use conditional visibility e.g. Where Blog Name = “…”.
Otherwise if you want a different vid eo for different blog articles you’ll need a different solution. Usually I’d go a programming route… store the vid URL in a link field or text field, and then use an embed with a <vid eo> element as custom code.
You’d need to figure out the HTML for what you want to create as a kind of manual background vid, and you’d need to host your vids somewhere like bunny.net.