Blue/Green deployments allow you to switch which environment of your project is the live version shown to public users. This allows you to make changes to your website in a non-live environment, and then "flip a switch" and instantly that becomes the live version people see.
This functionality is NOT currently available on GovCMS.
While it is under consideration, there is no ETA on when this will be available.
Alternatives
There are a few alternative methods you can use to achieve similar results.
Forklift
Forklift is the name given to the process GovCMS uses to move a website between environments. Commonly this is used when launching a new website, when a website is "forklifted" from an off-platform environment into the Master/Production GovCMS environment.
Forklifts can also be conducted between seperate on-platform GovCMS environments, such as migrating from a non-production branch like Develop or Test, into Master.
The primary downside with forklifts is that they need to be booked ahead of time, and have a window of time when they will be completed, rather than a set time that they will go live.
You can find more information on forklifts in the Knowledge Base.
Workspaces
Workspaces is a new module provided in Drupal core that allows you to stage large quantities of content and media in a preview environment on your live website prior to publishing.
This allows you to edit and view content as it will display on the live website before going live.
The primary downside with Workspaces is that you can't edit pages on the live version of the website once a draft is made in a Workspace, and that files uploaded in a Workspace are technically available to anyone with the URL.
These issues as well as a full description of how to set up and use the module is available on the Workspaces Knowledge Base article.
Workflows
Workflows is the most basic form of staging content, and allows you to save draft content that is only visible to authenticated users, before publishing the live version.
The Workflows module comes bundled on all GovCMS websites, and can be configured with additional steps as required.
You can find out more about Workflows on Drupal.org.