To be honest is the first time I hear it.
I consider your brother is either:
A- Someone that did not watch/read Bleach and only saw that scene, in his prejudgement thinks that the treatment the character received is only applied to those with dark skin (As if this was the typical 2000 yankee movies where black people were killed in horror movies)
B- Someone that is used that nowadays whenever a black character is on media, it just... doesn't get much involvement in situations like this, be it transformations, death or negative scenarios all together due to all the backlash and quick accusations of racism that happens especially at dumpster fires like Twitter.
Kubo, like most japanese people may not be that aware of whatever is going on in Western countries or at least not to the same level as the very same westerns, the same goes for other japanese mangaka or people in general that don't have this type of culture regarding skintone... like... kinda logical if there is not much immigration in the first place. Let's not forget that he is 45, there has been a lot of societal changes, movements and more through these past years that were quite quickly, he won't be like the new young mangaka debuting that probably knows a few gaijins and has seen more halfs than these older mangakas through their entire lives and is more open or simply more informed about western culture.
Kubo still, if I recall correctly is married. And considering his interest over Spanish language and culture, and there is a lot of speculation that it may be because his wife could probably be "hafu" or from mexican heritage.
Not to mention that characters in Bleach, regardless of skintone are being transformed into monsters, unlocking new forms, having badass powers or similar. All of them are being treated the same and not necessarily leaving a trope to only one type of character based on the skintone. There are Characters like Chad who address the fact of being half and how people bullied him for his skin but like the great badass he is he said "No, I take pride on who I am" and basically motivates Ichigo to be stronger too if I recall correctly.
Is been a while since I've watched Bleach, I would get spoiles from time to time from elementary school classmates about what was going on in the manga, and to be honest, as a latinamerican dark skinned gal I damn right loved Yoruichi and Chad who were already incredible in the series first seasons.