How do I code my own custom built blend tree node as seen in the Animation Controller Editor Window?

This question is just as open ended as this old one and you probably won't get a better answer. A whole node based editor from scratch is doable (I've done it) but it consists of way too many individual topics and the design is generally directed towards the purpose. Questions which just ask for google-result-like-answers (i.e. ask for suggestions, tutorials, ....) are too argumentative / subjective and are closed for that very reason.

Your question is simply too abstract and too broad to be answered clearly, just like the one I've linked. All I did in my answer was giving an overview over the IMGUI system of Unity. What you do with it is up to you. If you want to discuss UI design, editor tool design, ... you should head over to the forums. UnityAnswers is not for discussions of any kind. It's for clear techical questions that can be answered.

Therefore I will close the question.

ps: I just want to add that creating such a complex tool / tool set from scratch requires solid knowledge of Unity's IMGUI system, the Event system, how things are actually drawn, how interactions work, ... Also you should have a much more clear vision of where you want to go with this. What interactions you need and how you want to design them. Not in form of code but as really concrete concepts. The more complex a system the more important proper planing is. So, for yourself, write down specifically what you need and how it should work. Write it down in a way you can present it to your mother / father / friend so he would understand how to operate your system. The less clear your vision is how it should work in the end the more struggle you will have during the implementation.