According to the Oxford translation by Stephanie W. Jamison and Joel P. Brereton, the verse is referring to Dasyus. As the translators note, it's not referring to dark-skinned people but people that represent powers of darkness.
VII.5 (521) Agni Vaiśvānara
9 verses: triṣṭubh
The hymn invokes Agni as both the fire on earth and the sun in heaven.
More specifically, Agni is a tribal fire, which here represents an
aggregation of clans in the Pūru tribe (vs. 3). This hymn thus accords
with VII.19.3, in which Indra also supports the Pūru tribe, but
contrasts with VII.18.13, in which Indra supports Sudās, the enemy of
the Pūru. As Proferes (2007: 46–49) details, it is as a tribal fire
and as the sun that Agni is called Vaiśvānara, a name repeated in each
of the first five verses and then again in the last two (8–9).
The hymn begins with a statement of Agni's presence in heaven and on
earth, nurtured by both the gods and priests (vs. 1). In the second
verse Agni is called the master of both flowing and pooling waters, a
theme echoed in verse 8, the second to last verse, in which Agni is
asked to send the "refreshing drink," which might be the rain for
humans or the soma for the gods or both. Having descended to earth,
Agni's radiance extends to the Ārya clans (vs. 2cd). In verse 3 the
víśaḥ...aśiknīḥ "dark clans" are Dasyus (vs. 6), but they are not dark
because they are "dark-skinned," as the description is often
interpreted, but rather because they represent powers of darkness
opposed to the Āryas (cf. Hock 1999). Note that the brilliance of Agni
in 3cd breaks apart the "dark clans" and, as light, disperses
darkness. Or likewise in verse 6 Agni drives away the Dasyus,
providing a "broad light" for Āryas. As fire and sun, Agni extends his
light throughout the worlds (vs. 4). In verse 5, the middle verse of
the hymn, the poet concretizes Agni as the present sacrificial fire,
who guards and prospers the different communities and as both the
sacrificial fire and the sun, both of which appear in the early
morning as beacons of the day.
Bring forth a song to the mighty Agni, to the spoked wheel of heaven and earth, who as Vaiśvānara has grown strong in the lap of all the immortals through the watchful (priests).
Sought after in heaven, Agni has been placed on earth as the leader of the rivers and the bull of standing waters. He radiates outward
toward the clans descended from Manu: Vaiśvānara having grown strong
according to his wish.
The dark clans went breaking ranks, leaving their supplies, from fear of you, o Vaiśvānara, when you shone, breaking their strongholds,
blazing for Pūru, o Agni.
Heaven and earth, (each) in its three parts, follow your commandment, o Agni Vaiśvānara. You stretch throughout the two
world-halves with your radiance, blazing with your inexhaustible
Resounding ghee-rich songs—bellowing tawny mares—follow you, Agni, the lord of settlements, the charioteer of riches, Vaiśvānara, the
beacon of the dawns and of the days.
The good (gods) installed their lordship in you, for they find pleasure in your resolve, o you having Mitra's might. You drove the
Dasyus away from their home, o Agni, giving birth to broad light for
Being born in the highest heaven, at once you protect the fold on every side like Vāyu. Giving birth to living beings, you cry out,
doing service to their descendants, Jātavedas.
Send the heaven-bright refreshing drink for us, o Agni Vaiśvānara, o Jātavedas, by which you swell your bounty and broad fame for the
pious mortal, o you who grant all wishes.
Bind wealth that brings many cattle to our generous patrons, o Agni, as well as the prize of victory worthy of fame. O Vaiśvānara,
along with the Rudras and Vasus, offer great protection to us, o Agni.