In the Rig Veda, the main difference between the Aryans and their Dasa-Dasyu foes was not any race, but vrata (law, rule, rite etc https://sanskrit.inria.fr/MW/254.html#vrata).

Rig Veda 10.65.11 speaks of spreading Aryan laws all over the land.

Rig Veda 6.14.3 speaks of overcoming avrata (without vrata) Dasa-Dasyu (translated as 'fiend' here) with vrata. This along with Rig Veda 10.65.11 would refer to Aryanization.

Rig Veda 1.51.8 refers to discerning Aryans & Dasa-Dasyus (who are again mentioned as avrata)

So in Rig Veda, we see a battle between righteous law/rite/rule keeping Aryans and Dasa-Dasyus with no vrata. It is true that Dasa-Dasyus are associated with darkness and Rig Veda 5.14.4 mentions of Agni killing darkness and Dasa-Dasyus with his light.

It is obvious that the light of Agni or fire will destroy the darkness. So this need not to be taken as some racial reference of light skinned Aryans killing dark skinned Dasa-Dasyus. Dasa-Dasyus were associated with evil darkness precisely because they were evil & kept no vrata.

In Rig Veda 1.130.8 there is reference of krshna tvach or dusky skin associated with avrata and it is through the heavenly light this dusky skin is quelled by the Aryans. So certainly this is something supernatural and not a literal mention of dark skin.

In Rig Veda 9.73.3 asikni tvach or swarthy skin is quelled by maya (here translated as supernatural might ). So clearly this is something beyond the scope of humans. Also here again the dark skin is associated with avrata or riteless.

From the above passages it is clear krishna/asikni tvach or dark skin is closely associated with avrata or lawless/riteless. So it could be a metaphor to the evil darkness as mentioned in Rig Veda 5.14.4 which is associated with Dasa-Dasyus.

Rig Veda 7.78.2 explicitly associated evil with darkness.

Now who is an Aryan? As per Rig Veda an Aryan is the one who follows the path of Agni, established by father Manu. See Rig Veda 1.128.1 here

Rig Veda 10.11.4

Rig Veda 1.96.3

The Agni of Aryans was established by father Manu himself as per Rig Veda 1.36.19

So as per Rig Veda, an Aryan is the one who follows the ancient law of father Manu which is the path of Agni.