Currently, permalinks are set to /%postname%/.
I’d like to prefix “blog” before all standard posts.
I’m using several custom post types, each with their own rewrite rules in the setup.
How can I setup a custom prefix for my standard WordPress posts?
How do I set a custom base URL for standard posts?
did you try defining your perm structure like: /blog/%postname%/
If each one of your custom post types has its own slug defined, this should work like you are asking.