You actually can easily add a transparent header into a hero block and X-T9 allow it smoothly.
In order to accomplish that you need to create new header (you can copy / paste your existing header content) and set backgrounds as transparent. Name it something like “Transparent header”, then create a new template and start with a cover block. Add your Transparent header into the cover block before any other content. I just made a test: it works smoothly.
What is (currently) not easy with X-T9 in the example provided by @atsushi0514 is to have a fixed menu when you scroll down the page for further content. I just found a plugin named Sticky Menu & Sticky Header that claims doing exactly what you want. I didn’ test it yet, but since I’d like to implement something similar, I’m going to make some test.
I’ll let you know.
I’m sorry, I don’t speack Japanese (and used automatic translation), hopefully this is useful anyway…
申し訳ございません