"Blue" refers to the oarsmen and women in these first boats each earning their University Sporting Blue, an honour bestowed on sportsmen and women of each university who compete at the highest level in certain sports in Oxford/Cambridge competitions. Oxford is commonly referred to as the "Dark Blues" and Cambridge as the "Light Blues". Cambridge blue (the sporting variety) actually appears greenish.
The shades of blue used in the universities are both rowing colours of Eton College, whose alumni founded the respective boat clubs, and have been adopted by the universities as a whole.
At the annual gala after the race, the Boat Race Ball, members of Oxford's Dark Blue Boat traditionally wear dark blue blazers with dark blue trim. Members of Cambridge's Light Blue Boat wear a similar blazer in the light blue Cambridge colours. Lightweight crews wear Half-Blue colours, while the various reserve boats (listed below) wear more outlandish stylings.
Openweight men's reserves: Isis (Oxford) and Goldie (Cambridge)
Openweight women's' reserves: Osiris (Oxford) and Blondie (Cambridge)
Lightweight men's reserves: Nephthys (Oxford) and Granta (Cambridge)