The Jekyll’s introductory tutorial encourages you to create all pages with a header block that includes the key pair ‘title: A Page Title’. An unfortunate downside to this is that, under the default theme, every page is now added to your site’s auto-generated header - something that, as your site grows, you are unlikely to want to do.
Fixing this is simple; to exclude a page from the site’s header just omit your ‘title: A Page Title’ key pair.
… would add the page ‘Blog’ to the header and…
…would see it omitted.