Blog URL rewriting not working

I'm trying to rewrite /blog/post.php?s={postSlug} to /blog/{postSlug} but I just get a 404. I tried about every combination I can think of to get it to work but no joy.

My .htaccess file looks like this, with some rewrites to also remove the extension and force https. I've also tried removing all the extra rewrite stuff and just use the line for rewriting blog post urls, and that doesn't work either, just 404's. What am I doing wrong?

<IfModule mod_rewrite.c>
RewriteEngine on

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+)$ $1.php [L,QSA]

RewriteCond %{SERVER_PORT} 80 
RewriteRule ^(.*)$ https://www.mysite.com/$1 [R,L]

RewriteRule ^blog/([a-zA-Z0-9-/]+)$ /blog/post.php?s=$1 [L]s

</IfModule>