Why blackened metal turns up here

A star reads best against something dark, so celestial pieces often pair polished points with an oxidised ground. That oxidisation is a surface treatment on sterling, not a plating — it will lighten with wear and can be re-blackened by any bench jeweler. On gold, the same effect is usually a black rhodium plate, which is a coating and will need re-doing eventually. The piece's own page says which.