This plant is not the same as Solanum nigrum (cfr hua poporo). Both plants, however, may have given black pigment for tattooing. Bishop Jaussen:

"La Vergerette ... et le solanum ... son deux plantes bien différentes. Ici elles portent le nome commun de Poporo, parce que les indigènes tiraient des deux une couleur identique pour le tatouage." (Barthel)

A few preliminary remarks and imaginations:

1. From the following information I conclude this kind of poporo plant may refer to ti:

"Ms. E reveals yet another type of classification by listing ti (Cordyline fruticosa) and kape (Alocasia marorrhiza) in addition to the taro varieties as voluntary gifts from the fields of Teke. We are dealing with the contrast 'sweet vs. bitter'.

The creation chant has this to say about the origin of these two plants:

 In the first line, there is a reference to a former function of ti, because burnt ti leaves were used to produce the black dye for tattooing; in the second one, the reference is to the bad taste of so-called 'bitter taro'.

In 1957 kape was still cultivated in much the same way as dry taro. It is a type of food to be eaten during times of famine. According to Fuentes (1960:856), the tubers had to be kept in the earth-oven for 15 (sic) days in order to eliminate some of the poisonous components." (Barthel 2)

(Ti flowers. Ref. Wikidpedia)