It is certainly not Solanum nigrum. Have you matched the leaf characters of Solanum nigrum and Cayratia spp.? Solanum nigrum has simple leaves while Cayratia spp. have compound leaves.
Yes In sri lanka also we have this plant Cayratia trifolia (L.) Domin , a of family Vitaceae. we call it "Wal rat diya labu" Native liana. Occurance: open vegetation in the lowlands,near the coast behind the mangroves, in hedges and on waste lands. Flowering from June to November.
It is my suggestion to take detailed photographs as well as herbarium specimen. Give detailed measurements of all vegetative and floral parts as well ad stamen and carpel for taxonomic identification.
Cayratia trifolia commonly known as bush Grape, fox-grape, three-leaved wild vine or threeleaf cayratia is a species of liana plant native to Australia and Asia. It has black-colored berries, and its leaves contain several flavonoids, such as cyanidin and delphinidin. Hydrocyanic acid is present in the stem, leaves and roots