He has not a [sixpence] to bless himself with, i.e. in his possession; wherewith to make himself happy. This expression may probably be traced to the time when coins were marked with a deeply-indented cross. Cf. To keep the devil out of one's pocket.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Bless from Infoplease: