Hi OppaAi,
You are correct that Evaluators are not meant to be the “Blackboard” of StateTree. We also introduced Global Tasks that I would recommend using instead of Evaluators. Global Tasks run for the lifetime of the StateTree. These can be useful in exposing external data to the tree, configuring event listeners, and cleaning up the tree when stopping.
If you are wanting something similar to a blackboard, you should look into StateTree parameters. In 5.4, we introduced FStateTreePropertyRef which you can use to get a pointer to the data which allows changing the value of the parameter. There are limitations on the types that can work with the property ref and those are found in the StateTreePropertyRef header.
-James