Blæcmær was an OE masculine name, from the elements blæc ‘black, dark’ and mær probably ‘fame’ (von Feilitzen 1937: 203). It is not recorded outside DB, though the second element was common in OE personal names and the first occurs in several besides Blæcmær, all of which may well have been coined in the late Anglo-Saxon period.
Phillimore gives the name as Blaecmaer; a typographical error in both print and electronic versions of the Alecto edition makes it Blær.
von Feilitzen 1937: Olof von Feilitzen, The Pre-Conquest Personal Names of Domesday Book, Nomina Germanica 3 (Uppsala: Almqvist & Wiksells, 1937)
Spellings in Domesday Book: Blachemer
Forms in modern scholarship:
von Feilitzen head forms: Blæcmær
Phillimore edition: Blaecmaer
Alecto edition: Blær