Adding pages template selection locked

I’ve added a templates/blog.php and a blueprints/blog.php but when I add a page it is using my home template and i have a locked symbol by the dropdown. What have i missed?

In your blueprint you can specify the possible templates for new child pages. All options can be found in the documentation

Incase anyone else struggles with this:

My home page has child pages which are ‘projects’.

I want to add a blog page that has child pages ‘articles’

So for my home page I have

title: Home
pages:
  template: project
  sort: flip
fields:
  title:
    label: Title
    type:  text

This forces the project template to be selected.

In my blog blueprint I have:

title: Blog
pages:
  template: article
  sort: flip
fields:
  title:
    label: Title
    type:  text

You have to change the blueprints/site.php template to have pages set to true or force it to choose certain templates.