"Hung over" or "to hang over" means that the noun, in this case "the post-revolutionary haze," has permeated daily life, and is tangible, almost like a fog or a strong smell. Another example would be if there was a death in a family, and that one week later, it was said that the sadness still 'hung over' the household, meaning that the sadness was lingering and palpable.
To 'wear black' in this case means wearing black mourning clothing, which exists in many cultures. In many western cultures, black is worn to funerals exclusively, and members of the family wear black for a certain amount of time after the death.