A-sweartian

According to the Old English Dictionary:

a-sweartian

p. ode; pp. od To blacken, darken, to be made SWARTHY or black, obscured, darkened; denigrari :-- Ðæt gold biþ asweartod aurum obscuratur, Past. 18, 4; Hat. MS. 26 b, 8. a-sweartian