The masculine Old English name Blæc is not recorded before DB, where it appears both as a true forename and more frequently as a byname (von Feilitzen 1937: 203). The spellings Blac and Blach serve to distinguish this, the strong form of the name, from the weak form Blaca (Blache). Although the simplest of scribal errors would have confused the two names, in practice there is no possibility of mixing up the TRE landowners called Blæc and Blaca. Another instance of Blac, however, represents the equivalent Scandinavian name (ON Blakkr), usually in DB spelled Blacre (Blacher).
The adjective which forms the name was almost certainly OE blæc (‘black, dark’), referring in a byname principally to hair colour, rather than OE blāc (‘bright, shining; pale’) (von Feilitzen 1937: 203).
von Feilitzen 1937: Olof von Feilitzen, The Pre-Conquest Personal Names of Domesday Book, Nomina Germanica 3 (Uppsala: Almqvist and Wiksells, 1937)
Spellings in Domesday Book: Blac, Blach
Forms in modern scholarship:
von Feilitzen head forms: Blæc
Phillimore edition: Black
Alecto edition: Blæc