Wessex County, England during the Victorian Era
Tess of the D'Urbervilles takes place in the late 19th century (a.k.a., the Victorian period, or during the reign of Queen Victorian, 1837-1901), in an area of England to the southwest of London. Almost all of Hardy's novels take place in this same general area—ol' Thomas knew what he liked, and stuck with it.
The towns he mentions in Tess might appear in other novels, but they're all fictionalized versions of that real region of England. Hardy called his fictionalized version of this area "Wessex," so his novels that take place there are sometimes referred to as the "Wessex novels."
Tess's childhood home of Marlott (also in the county of Wessex) is in the "Vale of Blakemore or Blackmoor" (2.1)—Hardy repeatedly refers to it by both versions of its name. Why might that be? It's a fictional valley, anyway—why does he make up two versions of its name, and continually remind us that there are two accepted pronunciations?
Well, "Blakemore" is the older version of "Blackmoor" ("Blake" is Middle English for "Black," and "More" is the old spelling of "Moor"). It could be like the "D'Urberville"/ "Durbeyfield" connection. Maybe Hardy wants to remind us how much history there is in this place—it was a beautiful valley long before the Victorian period or Tess's time. So giving us the two names for the valley could just be a subtle way of reminding us that the valley, like individual families, has its own history and its own origins.