Robert,
That is a fantastic question. It can be done. If you’ll only be doing to that to some pages/posts then I would recommend using an older plugin called Art Direction. It allows you to apply custom styles on a per page/post basis.
Once you’ve installed then plugin, it’ll add a box down below your main content box when editing a page/post. On the right hand side is a spot where you can enter code that will only be visible on that specific page or post.
So on any page where you’d like to have the background be completely opaque, you’d paste this:
Once you publish your page that will set the content area to have a solid black background. It’s a pretty powerful, but simple plugin that you can use to really set some posts apart from the others.
If you have any questions or trouble, let me know,
Bill