Source: TW
Dasyus were originally Central Asian Farmers that the Indo-Iranians encountered. It’s possible some of the inversions of Aryan religion that Zarathustra did come from these people.
When the Indo-Aryans came to India, they used the word Dasyu/Dasa as a label for the local Harappan derived populations too.
Just like the word “Slave” comes from Slavs, but when you call someone a Slave you aren’t saying they are Slavic. Words can change meanings.
Yes, they were likely darker skinned than the Aryans but the verses abusing them for being dark are metaphysical often (darkness of soul, ignorance of the light etc)