"Diamonds obtain their colour from so-called "colour centres". They are single or mutliple non carbon atoms that replace carbon in the structure of the diamond, causing a disturbance in the structure and sometimes giving rise to the colour. The distinctive blue colour in diamonds is attributed to trace amounts of the element boron in the crystal structure. Minute traces of boron are required to create the colouration. Less than one boron atom per million carbon atoms is sufficient to produce the blue colouration".
Excerpt from the Natural History Museum website