I think even if race is not important, most people (maybe not all) would notice it as a normal description of the person. Like 'the blonde girl', you have 'the Black guy'. That's if everyone's not Black or if another characteristic that distinguishes him doesn't come to mind first, imho.
I think the main problem here is that blond hair is one variant within the group of people who are most often perceived and presented as the norm, while "the black guy" with no other identifying features given implies that being black is different enough to make him stand out from a crowd.
Consider reading a novel where black people were described in more detail and white characters were rare and always simply described as "the white girl."
Once in a while, it might make for an interesting change of pace or perspective. But if most of the novels you'd ever read were written with that assumption, it would likely get pretty old.
It's not offensive to notice that somebody is Black, lol. Back in grade school, I remember some kids used to whisper the word when talking about a Black kid!
Of course not, nor is it offensive to notice that someone is white. But think about how often a white person is referred to as "a white kid" in casual conversation (or in stories).
Of course, some of this is about numbers. In many places in the US, black kids are still very much in the minority, and it's human nature to focus on what are perceived as broad racial differences when someone is "the other," but to focus on smaller, more individualized differences when someone is a part of our own group.
It's not inherently bigoted or hateful, but it could be seen as a form of institutionalized racism, especially when it doesn't go away in environments with more diversity.
For instance, I teach at a community college where it's not a given that white students will be in the majority in my classes. Yet I still tend to think of white students as "that kid with the blond hair," or "that guy with the freckles," or "the petite young woman with the bangs," while I tend to think of people of other races as "the Asian guy with the buzz cut," or "That young, black woman with the freckles," or "The Latina woman with the braided hair."
I know I'm the product of my own upbringing, and I'm trying to change my perspective so white isn't the default assumption in my own mind.
In a book, of course, you have to portray your character's pov accurately, so having a white pov character think of someone who is black as "the black boy" isn't unrealistic, nor necessarily racist. But if this is going to be a part of my story, it's probably something I'd examine at some point, even if it's done in a subtle way (like maybe hearing oneself referred to as "the white girl" at some point and feeling a bit odd about it).