Solanum nigrum can also be purplish - see Richardson et al Weeds of the South East page 495. Plantnet distinguishes S. americanum (S. nodiflorum) from S. nigrum by the shininess of the berries and the number of seeds: Mature berry shiny; seeds usually more than 40 per fruit, 1–1.5 mm long - Solanum americanum Mature berry dull; seeds usually 20–35 per fruit, 1.8–2.2 mm long - Solanum nigrum. From ALA distribution records it is more likely to be S. nigrum - wait for the fruit.