I currently have an existing Preact JS site set up but want to add blog section.
So basically easy way to add new pages and routes.
I.e. blog/content-1, blog/content-2
The preactjs.com website is built exactly how you are wanting to build this. It's open source: https://github.com/developit/preact-www
Basically, fetch your HTML content via Ajax (however you'd like), then render.it as Virtual DOM using preact-markup.
For a real-world example, here's how preactjs.com does that: https://github.com/developit/preact-www/blob/master/src/components/content-region.js