It’s not just something esthetical (although I would have made exactly the same choice as you two 
), but the blue buttons also help visiblity and thus functionality big time. The black buttons do not stand out visually from the black rim very well, and combined with an indigo case, it’s only getting even harder to spot them at first sight. It’s a night and day difference since I equipped our local Community FP2’s indigo case with blue buttons.
On a different note, people have also played with giving their case and all new colour:
And here’s what one Fairphoner did with his rim earlier this year:
