Use below rule,
RewriteEngine On
# excluding www
RewriteCond %{HTTP_HOST} !^www
RewriteCond %{HTTP_HOST} ^blog
# excluding wp-admin
RewriteCond %{REQUEST_URI} !^wp-admin
RewriteRule ^ https://www.example.com/%1
Use below rule,
RewriteEngine On
# excluding www
RewriteCond %{HTTP_HOST} !^www
RewriteCond %{HTTP_HOST} ^blog
# excluding wp-admin
RewriteCond %{REQUEST_URI} !^wp-admin
RewriteRule ^ https://www.example.com/%1