For a product that is all about deployments, not having first class support for Blue Green deployments is.... odd.

Not a lot is needed. In fact you can't put in too much or it will be too opinionated.

Basically, all that is needed is something similar to your "tenant" system with a blue and a green line. But also show on the line which one is currently in use.

The "in use" flag will need a bit of effort as it can be a percentage or a list of users or both. But that is not too difficult to show.

The "in use" can be something that is set by deployment steps. It should allow a list of users (not required to be Octopus users) and also allow a percentage. Note that even though it is set in the step, it does not actually do anything to the deployed environment. (That can be a feature of a later revision.) Just having the UI and the Variable makes it so that it can be wired up to Octopus such that Ops Teams can see it an control it.

These are just my ideas, but it is weird that a tool that is all about deployments makes us coble blue green deployments together with tenants and a bunch of scripts.