A blue is an award of sporting colours earned by athletes at some universities and schools for competition at the highest level. The awarding of blues began at Oxford and Cambridge universities in England. They are now awarded at a number of other British universities and at some universities in Australia and New Zealand.

The first sporting contest between the universities of Oxford and Cambridge was held on 4 June 1827, when a two-day cricket match at Lord's, organized by Charles Wordsworth, nephew of the poet William, resulted in a draw. There is no record of any university "colours" being worn during the game.

At the first Boat Race in 1829, the Oxford crew was dominated by students of Christ Church, whose college colours were dark blue. They wore white shirts with dark blue stripes, while Cambridge wore white with a pink or scarlet sash. At the second race, in 1836, a light blue ribbon was attached to the front of the Cambridge boat, as it was the colour of Gonville & Caius College. These colours – light blue for Cambridge, and dark blue for Oxford – became the official colours of the two boat clubs, and through the rivalry of the Boat Race, the colours became inextricably linked with the universities and contests between the two.

Athletes at the University of Cambridge may be awarded a full blue (or simply a blue), half blue, first team colours or second team colours for competing at the highest level of university sport, which must include being in a varsity match or race against the University of Oxford. A full blue is the highest honour that may be bestowed on a Cambridge athlete, and is a much-coveted and prestigious prize. In general, the full blue standard is approximately that of being successful at a national level of student competition, and the half blue standard is that of being successful at county or regional level.

Once light blue had been chosen as the colour of Cambridge's Boat Club, the other university sport clubs followed suit, though out of courtesy would request permission from the Boat Club before awarding such a "blue". In the 1860s the three senior sports – rowing, cricket and athletics – were awarding blues, and the presidents of each formed an informal "Blues Committee" to oversee such awards. By 1880 a number of smaller clubs involved in varsity matches had successfully requested the right to award "half blues".

The criteria for awarding blues are different for men and women. Awards are made at the discretion of the men's and women's Blues Committees. The Men's Blues Committee is formed from one representative of each of the full blue sports, and the Women's from one representative of each of the full blue and half blue sports. Each committee meets to discuss issues relating to Cambridge sport.

In some sports with full blue status, the varsity match second team is awarded second team colours.

The awarding of a full blue often requires a person to fulfill a number of requirements in the same academic year, particularly in sports with discretionary full blue status. If, for whatever reason, an exceptional athlete is of or above full blue standard but does not fulfill all the requirements for the 'automatic' award of a full blue in any given year, the committee has the authority to grant an extraordinary full blue to that person subject to scrutiny of the particular case. The individual's case must be presented to the Blues Committee in person and must be backed up by substantial evidence and references. The award is unlikely to be made unless the person is of world-class or at least international standard.