For unknown reasons (probably scared that Hjor wouldn't have accepted his children's appearance), Ljufvina swapped them with a slave's blonde child and made Hjor believe that he was their son.

3 years later, it was decided that the truth needed to be addressed, as Geirmund and Havard started displaying at a young age "royal" qualities, such as being very smart and clever with words. After Ljufvina had confessed, Hjor went to meet his children personally, and accepted them within the royal family, giving them the title of "The Black-Skinned".

Despite it was common for Viking kings to marry many different women and have kids with them, Hjor did not marry any other woman other than Ljufvina, nor had any children with any other woman, implying that despite everything, they might have deeply loved each others.

Geirmund and Havard grew up to be viking sailors and warlus hunters like their father, and captained several expeditions in the seas of Northern Europe. There's very little about Havard, but Geirmund's fame as the "Black Viking" spread throughout the lands, as he created settlements in modern-day Ireland and England. After learning that his parents were ousted from the throne while he was away, Geirmund sailed again and created a settlement in Iceland that he ruled over.

Something about the "Black Vikings" I've discovered. Possibly the earliest account of an Eurasian mixed race family, and with royal titles no less, in modern-day Norway.
Source

The father, Hjor, was a king, descendant of the Augvald dynasty. Its forefathers claimed to be the descendants of the Gods. The mother, Ljufvina (literally meaning "the friendly lady") was a princess belonging to a Mongolian-Siberian tribe that settled in Bjarmaland, in modern day Russia. Because of their very dark skin, Ljufvina's tribe was called "the black-skinned". Their marriage was arranged as an agreement over the control of the walrus trade, which was very important at the time.

Ljufvina gave birth to Hjor's children when he was at a meeting with other Nordic kings. They were two twin boys, named Geirmund and Havard.