map.resources :controller and form_for

As far as I understand, you are trying to generate URLs with resource
name "blog" when actual controller is named posts. So the correct path
should be blogs_path (for your example) and not posts_path, and I
think it's a good idea that you specify it always. I don't know if you
can specify it globally.

But if it suits your need you can use :as. From rails guide:
map.resources :photos, :as => "images"
will recognize incoming URLs containing image but route the requests
to the Photos controller