Ugh, I’m not dumb, I know what pretty permalinks are, but obviously there’s no way for WordPress to just ignore pages that aren’t there (but take over posts, etc).
Well, I know that, but I have a site-wide 404 page defined by my own .htaccess, and it works everywhere except my /blog/ directory. Instead of duplicating code, I want WordPress to leave 404s alone and have ME handle them.