This is the inside of the nuts, they turn black after fermentation. Fermentation is necessary, because otherwise the nuts are poisonous.
id.wikipedia.org/wiki/Kepayang#/media/Berkas:Keluak_(Pangium).jpg

They are used in certain Indonesian dishes such as Rawon and Brongkos. The dish becomes black, the nuts have a slightly bittery/chocalately taste.