The Black Guan is primarily frugivorous, feeding on fruits from trees and shrubs in the montane forests it inhabits. A study in Monteverde, Costa Rica reported Black Guans feeding on fruits from twenty six different species of plants (Wheelwright 1984). These fruits range from large-seeded members of the avocado family, Lauraceae, to the diminutive fruits of Urera elata, a treelet in the nettle family.
Black Guans have also been observed eating the leaves of Hamelia patens, a treelet in the family Rubiaceae (Joyce pers. obs.).