I love the fact that the headcanon of the Blacks being uncanny and creepy, makes 100% sense in canon.
They're described as beautiful with sharp jawlines, pale skin, dark hair, dark eyes, and many being tall with "elegant structure." They're these gorgeous people, and hold themselves to a standard of being this Wizarding World Royalty.
However, they're fucked up at the same time. They have the crazy eyes. It's a coin toss on if they're insane or just severely traumatized; regardless of which, you get this weird feeling around them, because you know something is wrong.
They all look alike, too much a like; there's not enough genes in the pool to make them look too different after all. They look normal alone, but when you put them all together, this uncanny valley feeling settles in when you look at them all enough. Cousins looks like they could be siblings, children are nearly identical to their parents. No one looks the age you expect, everyone is either younger or older than you thought, because no one really looks.. right.
They have a ton of money, power, and connections, so if you look at them wrong for too long, something could happen to you. Just the power alone makes many fear them, but along with the unnerving appearance, being around them must have been complicated for many people.
They're also intelligent, raised with the finest tutors and access to ancient knowledge many could only dream of. If they can't spook you with looks or power, they can outsmart you and plot your downfall with magic alone, no connections involved.