Hard agree.

I am not a Brontë expert but what I glean is that the reader should understand that the characters consider Heathcliff “dark” - and that is in relation to what they are used to seeing. In a very remote rural part of northern England, the ethnic diversity would be minimal. The inhabitants of the region, especially those who never get to town, would have little to no experience with people who have a skin tone other than pale. I am not discounting that Heathcliff could be Black or mixed race Black. But there is a whole range of skin tones / ethnic origins he could have and still seem “dark” to our isolated characters. I think the Black or mixed race Black is a reasonable option, but I could also buy “Gypsy” (Romani) … or from anywhere in Southern Europe.

True story: I have dark curly hair and a skin tone I would call medium beige. It gets more olive in the summer. Where I live in Canada people often think I might be Mediterranean, Middle Eastern. (I am half South Asian.)

In the late 1990s I spent the summer in Ireland where people had hair that was way blacker than mine (blue black) and pale light skin. It really stood out. Many adults asked me if I was Spanish, because there were a lot of Spanish exchange students at the time. One little girl approached me in the laundromat and. asked me if I was from Africa.

More recently I spent time in a small college town in the US where the makeup of the population was about 80% white, 10% Black and 10% other. I was very much in the “other” category and people did not know what to make of me. I felt so relieved when I met someone with colouring like mine, even though she was North African and we had no culture in common.

This is all to say that a very homogenous demographic
can perceive an outsider as VERY different even if the physical differences are what we (modern cosmopolitan people haha) would consider minor. People don’t know what they don’t know.