The Rhombeus is the Black Piranha. There is no such a species as the Niger ( Niger's Piranha is the Serrasalmus Rhombeus a.k.a Pirana NEGRA..
Serrasalmus Rhombeus Highback ( mostly found in Peru turned Jet Black in color when adult ) I have 17" Highback and is completely black. I also have 3 nine inch rhombeus and they are dark grey just turning black. Serrasalmus Rhombeus is white in color when young with yellow and red coloring on the chest.