The best defined images of Nubians are probably these tiles, and this drawing of a Nubian from the Belzoni illustration of the tomb of Seti (the original sadly very damaged by moisture) is probably the best known. As can be seen, the standard depiction is of a jet black skin, afro hair, a big hoop earring, and totally negroid faces with big lips, wide nose and prognathic in profile.
This is probably closer the appearance of the Southern Nubians, as studies of a Northern Nubian cemetery (Semna South) shows about 60% Eurasian ancestry, which makes sense as the ancient Egyptian Southern border area (Luxor) shows at about 70% Eurasian. In case you are interested, the hair study is here, and the DNA study is here.
A recent study of the Sudan suggests 40% Eurasian from the Y chromosome, although it’s probably less overall, so the percentage of Eurasian ancestry in these people probably dropped off pretty quickly as they moved South.
I’m assuming it’s the Southern Nubian ‘type’ depicted as a kind of understandable stereotype in the artwork, as a Northern Nubian probably didn’t look definably different to a Southern Egyptian, and would not be recognisable as a Nubian if represented exactly in the artwork. A similar thing is seen in the depictions of Libyans (North Africans) who are generally given a very white skin and often fair hair, when in reality they would have been just as heavily tanned and dark haired as the average lower Egyptian, and hard to tell apart without cultural trappings.
Two Nubians (ambassadors I think). Nubian archers, From the tomb of Mesehti, a prince from the region of Assiut (Middle Dynasty, about 2000 BC).
Nubian slaves, the Great Temple, Abu Simbel, Egypt.