Firstly go to https://www.google.com/recaptcha/admin. Sign up if needed and then go to the "Register a site section".

Label: Anything you wish.

Choose the type of reCAPTCHA: Recaptcha V2

Domain: Your blesta domain eg: yourdomain.com or billing.yourdomain.com

Accept the terms and save.

You can then go to the table above and click the row for your label.

You will see this: Adding reCAPTCHA to your site

Click on keys and then add the two keys to Packages > Order forms > Settings > Human verification