Completely agree on both counts. We do usability testing, including with keyboard-focused advanced users.

But usability testing with blind users presents some unique challenges. A past org I worked at ran some usability studies with blind users [1] and while I was only tangentially involved in that project it seemed that subject recruitment and observations were much more complex than typical usability studies. I haven't managed to run a usability study with blind participants at my current org though we have discussed ways we could recruit blind users for studies -- our software is complex enough that we'd need someone who is both blind and a prospective user of our software.

[1] https://www.bloomberg.com/ux/2018/08/28/visually-impaired-wo...