The XC you are seeing is actually part of the ornamental design. This is Great Britain 1shilling. I don't have my catalog in front of me but look around the 1880 era and you should find it. The blue is not part of the stamp but due to postmarking or other markings done later, but not part of the actual stamp design colours.
Date of Issue: 1887 Denomination: 1 Sh Perforations: 14 Sheet Size: 240 (½d-3d, 5d, 6d and 1s), 80 (4d and 4½d, 9d) and 96 (10d) # Issued: 138,600,000 Design: Height/Width: Theme: People on Stamps/Royals/Victoria, Queen
The blue is likely part of a postal marking that indicated registered mail in many countries: conspicuous large crosses, running across the entire envelope, made with blue crayon (occasionally in other colors). This marking was common in Britain and her colonies, but was used in other nations as well.