Sorry [no-one ;)], I figured it out:
- /category/ can be removed in Yoast SEO by checking ‘Strip the
category base from the category URL.‘ - The other part ‘graphic-design-blog’ was displayed because all of the
categories were had the parent ‘blog’ which had the slug
‘graphic-design-blog’. I set all the sub-categories to have the parent: none.
Embarrassingly simple 🙂