A dark launch is a deployment strategy where a new version of a service is deployed in production and traffic is routed to it without returning responses to users. This allows testing of the new version's performance, error handling, and compatibility with production load. Monitoring is crucial during a dark launch to ensure its success, prevent infrastructure issues, and catch any unexpected behavior or errors. To monitor a dark launch, you should track service level indicators (SLIs), identify unexpected responses, analyze logs, run automated tests, and set up dashboards to visualize performance metrics alongside other key metrics. Additionally, you need to ensure that your infrastructure has ample capacity to handle the new version, spot unintended interactions with persistence layers, and get comprehensive visibility into your dark launches. Tools like Datadog can help simplify this process by providing a unified visualization, alerting, and analysis platform for monitoring all data from your dark launches.