It is likely that he was Indo-Aryan, descended from the Scythians, the Sakya clan.
Genetic research shows that blue-eyes originated around the northwest coasts of the Black Sea (http://www.dailymail.co.uk/sciencetech/article-511473/All-blue-eyed-people-traced-ancestor-lived-10-000-years-ago-near-Black-Sea.html) which is the same region that the Scythians were allegedly from.
It is also likely that the first people with blue eyes were all black-haired rather than light-haired as ancient Europeans are found to have dark-hair and blue-eyes, indicating that light hair came separately from blue-eyes and maybe more recently as well (http://www.livescience.com/42838-european-hunter-gatherer-genome-sequenced.html). Where light hair comes from is still a mystery, it may have come from the Neanderthals. Modern day Europeans are likely not Aryans but Aryans admixed with Neanderthals or mutated into something else, which explains their features.
It is not that uncommon to see people in modern day India and Nepal with black hair and blue eyes just as Siddhartha Gautama was described.
But it is also important to note that The Buddha was against the caste system and someone's ethnicity does not make one an arahant. An arahant is someone who is free from mental effluents, pains, taints, hankers, fermentations (Âsavas).
He used many arguments against the caste system in the Assalayana Sutta (MN 93) mainly arguing that good deeds are good for anyone of any caste and that bad deeds are bad for anyone of any caste, also using a clever analogy that anyone of any caste can make a fire and the fire would still be a fire with a flame regardless of the person who made the fire.
"No, Master Gotama. The fire made by those born from a noble warrior clan, a brahman clan, or a royal clan... would have flame, color, & radiance, able to do whatever a fire might be needed to do. And the fire made by those born from an outcast clan, a trapper clan, a wicker workers' clan, a cartwrights' clan, or a scavengers' clan... would have flame, color, & radiance, able to do whatever a fire might be needed to do. For all fire has flame, color, & radiance, and is able to do whatever a fire might be needed to do." (Assalayana Sutta, MN 93)