There were a bunch of steppe tribes that were nomadic and would raise various steppe animals/raid the area just west of china. They would occasionally form confederations and fight/get assimilated by the chinese and the persians. The most successful of these tribes over the years (ie the turks and the mongols) had empires we are familiar with that stretched from asia to byzantium. But the lesser known and weaker tribes would get pushed out west as the others conquered and so they would go west in the step until they hit protected european settlements. Thus we get the bulgars, huns, etc. So they were asian initially as you can tell by the hilariously unflattering byzantine descriptions of them but they were heavily mixed as they moved t through the middle east and eventually became whiter by integrating with the local europeans. Interestingly the chinese didnt like these tribes either because they were raiding barbarian so its fun to read about major empires trashing on these smelly uncivilized long haired horsemen on both ends of the "known world."