Thank you anon!!!
The TLDR is:
1) I based his designs more off the common portraits/images we have of him, and in most of the very few screen depictions of him, he's brunette. We don't know the reality irl so it's just personal choice.
2) it contrasts better with the blonde/redhead Angevins
3) there's already a lot of Blond French bishie designs out there so I wanted to do something different so it wouldn't get mistaken for other popular characters

I go into more detail here heheh.
https://angevinyaoiz.tumblr.com/post/752602161425334272/haha-thanks-for-appreciating-my-art-yeah-its