Version B is released alongside version A, then the traffic is switched to version B. Also known as red/black deployment.
The blue/green deployment strategy differs from a ramped deployment, version B (green) is deployed alongside version A (blue) with exactly the same amount of instances. After testing that the new version meets all the requirements the traffic is switched from version A to version B at the load balancer level.
You can apply the blue/green deployment technique for a single service or multiple services using an Ingress controller: