Blue in Old English represents the first thorough investigation of an area of the colour semantics of Old English, and the methodology developed for this study is believed to be appropriate for researching the colour semantics of any language which survives only in recorded texts. By means of a collection of in-depth word-studies, which suggest new interpretations of many well-known passages, an understanding of how blueness was described in Old English is developed. The approach is interdisciplinary, using evidence from subjects such as botany, manuscript illustration, etymology, early technologies, and others. The conclusion contradicts certain previously held views on Old English colour, and presents a hitherto obscured sociolinguistic picture of differing language use among various groups of Old English speakers.