Add /blog to the “post” post type url

You can do this by adding this line of code to your Register_post_type $args Function array:

$args array

'with_front' => false,

this will remove the blog from the front of the CPT that you add this line.

Remember to flush permalinks after you add it and save.

from the codex:

‘with_front’ => bool Should the permalink structure be prepended with
the front base. (example: if your permalink structure is /blog/, then
your links will be: false->/news/, true->/blog/news/). Defaults to
true.

you can see further reference here:
https://codex.wordpress.org/Function_Reference/register_post_type

or just ask!