"Blue blood" and "true blue" being originally Spanish phrases, the old families of Spain who trace their pedigree beyond the time of the Moorish Conquest claiming that they have venas ceruleas , whereas the blood in the veins of the common people is black" (Cook and Wedderburn, XXVII, 122).
Note how this piece of verbal play enables Ruskin to figure both as a red in the present letter and as a true-blue Tory in Letter 10. For the meaning of purple and the colour of blood, see also 'The Work of Iron', p. 121, and my notes 9 and 10 on pp. 324-5.