Your instincts about red-green-refactor being hard to follow in a blog post that’s not about TDD itself is correct.
I think that doing the step-by-step process of TDD is really valuable when you are trying to make a point about TDD (and definitely good practice while you’re working), but I think it can make for a distracting blog post if you are trying to cover another topic. I would post your tests at the beginning or end of the post (alongside the code, so maybe final code and final tests?), to show some of your testing chops, but I would focus the meat of your post on ActiveModel.
ActiveModel is a great topic, look forward to seeing the post!