You can call them whatever you like. Here at my house the 4 baby blacks are known as the "Tilleys" Black, Blue or Tilley it still boils down to, they are all the same Osteoglossum ferreirai.
As with all Arowanas; be they Silvers, Reds, Golds or Blacks...feeding a high quality diet especially high in Karotine will enhance the fishes coloration.