The Hawaiian name popolo actually refers to a number of species of plants in the Solanum genus, a member of the nightshade family. They are annual or perennial herbs, up to 4 feet tall. There are four species of Solanum native to the Hawaiian Islands. One species, which may have been an early Polynesian introduction, is S. americanum with reportedly edible fruits. Three endemic species are pōpolo kū mai (S. incompletum), pōpolo (S. nelsonii), and pōpolo ʻaiakeakua (S. sandwicense); all of which do not have edible fruits. S. americanum occurs in a wide variety of habitats including coastal forest, wet forest, pastureland and disturbed roadsides from sea level to over 7800 feet on all of the main islands.
Popolo is an important medicinal plant reported to treat respiratory problems and also for sore muscles, tendons, and joints. It is often used by itself or mixed with other ingredients. In addition, the blackish purple fruit and green leaves were used for dyes.
The fully ripe black berries of the pōpolo (S. americanum) were enjoyed by early Hawaiians; however, the green berries are poisonous, containing solanine along with other nightshade toxins. The leaves were wrapped in ti leaves, cooked in an imu and eaten in times of food scarcity.
Some other edible and favorite crops in this nightshade family include tomato, eggplant, peppers, cape gooseberry (pohā berry) and tomatillo.
The following two websites give information about Hawaiian medicine and popolo in particular: http://www.medicineatyourfeet.com/Solanum_americanum.pdf and hawaiianmed.com. I am not familiar with these websites and cannot vouch for their accuracy.