Huns became mixed when they got to Europe [as a hodge-podge of all the tribes they swept before and with them]. The Hsiung-nu tribal group Sima Qian was describing was not particularly mixed - a Chinese historian would surely make a prominent note of that.
I've read that there are many poems in Chinese lamenting the green eyes of the Han emperors. Also, that the Han emperors had red-headed people as their 'magic' makers.