Actually, no, it isn't, for all the reasons that @JediShark gave you.
What you have described is the epitome of a pop-up, and it's a real pain for users. As someone who has lately been spending much of my time re-coding sites to make them fully accessible to people with disabilities, I will also add that such popups are a total nightmare for anyone unable to use a mouse. (Whereas a blind user, for example, has no difficulty using the Back button, despite what you say.)
The only popups I consider acceptable are those that replace browser alerts, because they make such alerts consistent across all browsers. But, even then, I'd expect them to be coded with accessibility in mind.