Rig Veda II.20.7
स वृ॑त्र॒हेन्द्र॑: कृ॒ष्णयो॑नीः पुरंद॒रो दासी॑रैरय॒द्वि ।
अज॑नय॒न्मन॑वे॒ क्षाम॒पश्च॑ स॒त्रा शंसं॒ यज॑मानस्य तूतोत् ॥७॥
sa vṛtrahendraḥ kṛṣṇayonīḥ purandaro dāsīrairayad vi | ajanayan manave
kṣāmapaśca satrā śaṃsaṃ yajamānasya tūtot ||
Translation from the Rig Veda by Stephanie W. Jamison and Joel P. Brereton is as follows:
Smasher of Vr̥tra, splitter of fortresses, Indra razed the Dāsa
(fortresses) with their dark wombs. He gave birth to the earth and the
waters for Manu. In every way he makes the sacrificer’s laud powerful.
Traditional translator Sri Sayana laid a path, which was followed by many Western translators of Rig Veda, restricting the meanings of Rig Vedic mantras to narrow Karma Kanda.
Sri Aurobindo says on Sri Sayana as follows:
...it is the central defect of Sayana’s system that he is obsessed
always by the ritualistic formula and seeks continually to force the
sense of the Veda into that narrow mould.
Thee word Dāsa in the above verse can be understood from the article of Sri Aurobindo .
Sri Aurobindo explains Dasyus or Dāsas as follows:
We have seen, not once but repeatedly, that it is impossible to read
into the story of the Angirases, Indra and Sarama, the cave of the
Panis and the conquest of the Dawn, the Sun and the Cows an account of
a political and military struggle between Aryan invaders and Dravidian
It is a struggle between the seekers of Light and the powers of
Darkness; the cows are the illuminations of the Sun and the Dawn, they
cannot be physical cows; the wide fear-free field of the Cows won by
Indra for the Aryans is the wide world of Swar, the world of the solar
Illumination, the threefold luminous regions of Heaven.
Therefore equally the Panis must be taken as powers of the cave of
Darkness. It is quite true that the Panis are Dasyus or Dāsas; they
are spoken of constantly by that name, they are described as the Dāsa
Varna as opposed to the Arya Varna, and varṇa, colour, is the word
used for caste or class in the Brahmanas and later writings, although
it does not therefore follow that it has that sense in the Rig Veda.
The Dasyus are the haters of the sacred word; they are those who give
not to the gods the gift or the holy wine, who keep their wealth of
cows and horses and other treasure for themselves and do not give them
to the seers; they are those who do not the sacrifice. We may, if we
like, suppose that there was a struggle between two different cults in
India and that the Rishis took their images from the physical struggle
between the human representatives of these cults and applied them to
the spiritual conflict, just as they employed the other details of
their physical life to symbolise the spiritual sacrifice, the
spiritual wealth, the spiritual battle and journey.
But it is perfectly certain that in the Rig Veda at least it is the
spiritual conflict and victory, not the physical battle and plunder of
which they are speaking.
Coming to the question part - the inner meaning of the word kṛṣṇayonīḥ, we have another meaning than the known meaning of dark wombs of Stephanie W. Jamison and Joel P. Brereton.
It can also mean Dark Origin/born out of darkness.
The power of the Almighty God is called Indra (epithet).
Indra destroys the power of ONE's intentions/weaknesses born out of ignorance (darkness), which hinders the deliverance of Illumination (cow/waters) or SELF REALISATION, which is hidden within ONESELF (cave) to humans.