Here is what i found over the net, unforunately it was in wikipedia, but i remember reading egyptologist/nubiologist Yurco book titled "Black Pharaoh" and i remember this particular quote from his book.
Various pharaohs of Nubian origin are held by some Egyptologists to have played an important part towards the area in different eras of Egyptian history, particularly the 12th Dynasty. These rulers handled matters in typical Egyptian fashion, reflecting the close cultural influences between the two regions.
...the XIIth Dynasty (1991–1786 B.C.E.) originated from the Aswan region. As expected, strong Nubian features and dark coloring are seen in their sculpture and relief work. This dynasty ranks as among the greatest, whose fame far outlived its actual tenure on the throne. Especially interesting, it was a member of this dynasty that decreed that no Nehsy (riverine Nubian of the principality of Kush), except such as came for trade or diplomatic reasons, should pass by the Egyptian fortress and cops at the southern end of the Second Nile Cataract. Why would this royal family of Nubian ancestry ban other Nubians from coming into Egyptian territory? Because the Egyptian rulers of Nubian ancestry had become Egyptians culturally; as pharaohs, they exhibited typical Egyptian attitudes and adopted typical Egyptian policies. (Yurco 1989) 
In the New Kingdom, Nubians and Egyptians were often so closely related that some scholars consider them virtually indistinguishable, as the two cultures melded and mixed together.
It is an extremely difficult task to attempt to describe the Nubians during the course of Egypt's New Kingdom, because their presence appears to have virtually evaporated from the archaeological record. The result has been described as a wholesale Nubian assimilation into Egyptian society. This assimilation was so complete that it masked all Nubian ethnic identities insofar as archaeological remains are concerned beneath the impenetrable veneer of Egypt's material culture. In the Kushite Period, when Nubians ruled as Pharaohs in their own right, the material culture of Dynasty XXV (about 750–655 B.C.E.) was decidedly Egyptian in character. Nubia's entire landscape up to the region of the Third Cataract was dotted with temples indistinguishable in style and decoration from contemporary temples erected in Egypt. The same observation obtains for the smaller number of typically Egyptian tombs in which these elite Nubian princes were interred