We do enable blue/green deployments, but there isn't a "one-size-fits-all" method to perform them; instead, we have a three-step process to come up with design patterns for any application. What we will do here is create a state machine for the deployments, with Terraform being the tool to move between states.
First, we break the blue-green deployment down into desired states. Second, we determine what mechanisms are needed to make the changes between the states. Third, we put it all into a single flow using a combination of terraform functions, cloud components, and/or scripts as needed.
So let's walk through a simple application deployment: a 2-tier web application, with a single instance, being routed to via a Load Balancer. Note that the site's DNS name is pointing to the Load Balancer.