"pleasing inactivity, sweet idleness," 1814, from Italian, literally "sweet doing nothing." The Latin roots are dulcis "sweet" (see dulcet), facere "to make, do" (see factitious), and nec entem, literally "not a being."
This phrase, frequent enough in English literature, does not seem to occur in any Italian author of note. Howells says that he found it current among Neapolitan lazzaroni, but it is not included in any collection of Italian proverbial sayings. [Walsh]
In reference to dark eyes, often as a mark of beauty, from 1660s. Black-eyed is from 1590s of women, of peas from 1728. The black-eyed Susan as a flower (various species) so called from 1881, for its appearance. It also was the title of a poem by John Gay (1685-1732), which led to a popular mid-19c. British stage play of the same name.
All in the Downs the fleet was moored,
The streamers waving in the wind,
When black-eyed Susan came aboard,
"Oh! where shall I my true love find?
Tell me, ye jovial sailors, tell me true,
If my sweet William sails among the crew?"