Black mourning dress reached its peak during the reign of Queen Victoria (1819-1901) of the United Kingdom in the second half of the 19th century. Queen Victoria wore mourning from the death of her husband, Prince Albert (1819-1861), until her own death. With these standards in place, it was considered a social requisite to don black from anywhere between three months to two and a half years while grieving for a loved one or monarch. The stringent social custom existed for all classes.
V& A Museum, item no T.73-2010. Mourning Dress, Scotland (Britain): c1823-1825, silk velvet, with silk satin piping and appliqué. "The death of Princess Charlotte in 1817, the only child of George IV plunged the whole country into mourning & set the high standards for mourning dress"