Heathcliffe's origins are hazy. He is found on the streets of Liverpool by Mr Earnshaw and brought back to Yorkshire.
Liverpool has had a well-established Black population since the early 18th century. To a rural Yorkshire person a child of mixed race might indeed look like a 'dark-skinned gypsy'.
The book's characters speculate about his racial origin. He might a American or Spanish or the offspring of the Emperor of China and an Indian queen. Or a the child of a 'Lascar', which was a sailor from the Indian subcontinent. (Conan Doyle also mentions Lascars.)
Heathcliff might be any of those, or simply a Liverpool mixed race child, whom Earnshaw probably had no right to take away from his admittedly inadequate home. We will never know!