Blueprint

Add your SEO field to the blueprint like this:

fields:
  seo:
    type: seo

Custom field key

If you want some other key than seo it's really important to add an option to your config.php:

c::set('seo.field.key', 'seo');

You can add a label on it if you like, but you don't need to.

To not repeat yourself in every blueprint I highly recommend to use Global Field Definitions.

Add a file: /site/blueprints/fields/seo.yml

In that file, add this:

type: seo

Call it from the blueprint like this:

fields:
  seo: seo