This isn't the exact date, but it's the first one I have recorded of the origin of the idea.
I was at the planning stage of building my own personal blog. My personal website was already built using Jekyll, I liked it better than any other options and kept building on top of it. I like to keep things simple so any kind of CMS would be overkill.
This got me thinking about the options that non-web-developers have for creating their own blog, and thought that there could be small space for a solution that combines the simplicity of Medium and the flexibility of Wordpress. Something like Carrd but for blogging. In fact, Carrd was a big inspiration, so much that the first name I came up with was Blogrr.
It was still a very fragile idea though, and I didn't think it could work since there already many blogging solutions.