I also asked this question years ago. There is no definitive answer. Look at the descriptions listed on the ship manifests on EI and you will see it varies. I collect Czech/Slovak Dolls. Some are blue eyed/blonde and others are Dark brown hair/brown eyes. My family line is all dark hair/eyes, but others are the blonde/blue......
I'm sure there is a historical reasoning behind this, but that is not my field of expertise!