i think any mind-mapper software is fine.
i use one called scapple because it is one-time purchase, cheap, and offline. But anything where you can connect some nodes and add text should be sufficient.
If you are starting from square one my personal recommendation is dont use blackboard yet. It brings in a lot of new concepts and terminology and you have to jump through more hoops to handle communications. In other words, it’s not exactly beginner friendly.
It is easier to get a basic AI setup with a simple state pattern in blueprints. You can actually do quite a bit that way. The result is that you dont need to learn any new language and you can handle communications between classes in the typical familiar way.
If you don’t know what a state pattern is and how to implement it in blueprints, try searching youtube there is a few great tutorials.
As for the planning, i just try to identify the states and then know when one transitions to another. Just describe it out in plain language and then translating to code isn’t too hard.