I've been told that each product team is supposed to add CloudFormation support themselves. For some reason, it's not treated as a high priority and often lands on the laps of the core CloudFormation team.
It may turn out that the tools that CloudFormation team provides don't make integration easy, especially if the operation takes longer than 15 minutes (meaning they can't implement support via a single lambda invocation as an under-the-hood custom resource).
You can also invoke a custom resource via SNS and then the SNS topic is subscribed to an API endpoint.
But it’s hard to believe if the team responsible for the blue green deployment functionality developed an API endpoint to do it, they couldn’t just hand it to the CF team to call. At the end of the day that’s all CF does. Call APIs based on the different lifestyle events as far as how it actually creates resources.
It may turn out that the tools that CloudFormation team provides don't make integration easy, especially if the operation takes longer than 15 minutes (meaning they can't implement support via a single lambda invocation as an under-the-hood custom resource).