Hi MH THemes,
thank you for your reply! You are correct that by blog page I refer to the posts page as set in settings->reading or the customizer. I did not know that the posts page is considered an archive, but I am unsure of how that affects what headers/featured images you can or cannot have.
I have set a featured image for all pages, and when I’m in the page editor it shows the correct featured image on every page. However, in viewing mode the featured image on the posts page just doesn’t show up. For the life of me I cannot understand why this is. It seems to me like there would be no valid reason for not enabling featured images for the posts page. And it worked perfectly when I first launched the site and the posts page was by default the front page.