You can replace your current code by this one in your htaccess
RewriteEngine On
RewriteBase /
RewriteCond %{HTTPS} off
RewriteCond %{REQUEST_URI} !^/blog_rss\.php$ [NC]
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L,QSA]
RewriteCond %{HTTPS} on
RewriteRule ^(blog_rss\.php)$ http://%{HTTP_HOST}/$1 [R=301,L,QSA]
EDIT: looks like %{HTTPS}
is not recognized on some servers, which is causing an infinite loop.
Try with %{SERVER_PORT}
(if default http port is still 80 and ssl port is 443)
RewriteEngine On
RewriteBase /
RewriteCond %{SERVER_PORT} 80
RewriteCond %{REQUEST_URI} !^/blog_rss\.php$ [NC]
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L,QSA]
RewriteCond %{SERVER_PORT} 443
RewriteRule ^(blog_rss\.php)$ http://%{HTTP_HOST}/$1 [R=301,L,QSA]
You could also try with your initial syntax
RewriteEngine On
RewriteBase /
RewriteCond %{ENV:HTTPS} !on [NC]
RewriteCond %{REQUEST_URI} !^/blog_rss\.php$ [NC]
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L,QSA]
RewriteCond %{ENV:HTTPS} on [NC]
RewriteRule ^(blog_rss\.php)$ http://%{HTTP_HOST}/$1 [R=301,L,QSA]