While Vikings were in fact white, there was no such thing as race purity in Norse culture.
The term Átt in norrœnt mál (Old Norse) can either mean race or family. It is yet another indication that race or blood line were not necessarily relevant in Viking times, allowing for far reaching commerce and settlements. Emphasis instead was on family, tribes, and overall cultural belonging. This Viking specificity led to the occasional breeding with non-whites, making racial purity simply genetically impossible.
Occasional Multi-racial Vikings
According to Landnámabók (Book of Settlements), one of Iceland's first settlers, over 1,200 years ago, is named Geirmundur Heljarskinn Hjörsson. The nickname, Heljarskinn, translates to "skin like hell”. In context, it does not refer to black skin, but instead to a man who isn’t as white as his peers. This is because Geirmundur Heljarskinn isn't completely white. He is half Mongolian, and the product of the union of his father, Norwegian king Hjör Hálfsson, with a Mongolian woman, during an expedition in Bjarmaland (Siberia). Iceland's part Mongolian origins are further confirmed in the country’s gene pool, and explain why some ethnically Icelandic people, such as Björk, have Asian physical features.
Æsir of un-pure blood
Þórr's sons, Magni and Modi, are of "impure" blood, because they have been mothered by Járnsaxa, a giantess. Yet, they are still considered so mighty and divine, that not only are they an integral part of the Æsir, but they are to inherit Mjölnir from Þórr after Ragnarøkr to become the defenders of gods and men alike.
Overall White Vikings
Of course, none of this is meant to even remotely suggest that Vikings weren’t white overall. Cultural specificities were normally associated with race. As a result, Viking predominantly bred with the same race, because it so happens to typically be the same culture. There is also the aspect of life in the north requiring white skin for Vitamin D absorption, and thus, survival. Inter-racial breeding was therefore the exception, not the norm. Such rare exceptions, however, make racial purity among Norse people simply genetically impossible.