In the Rigveda (6-9-1), the terms kṛṣṇa and arjuna are found in a slightly different context than we all see in the Mahābhārata.
कृ॒ष्णमह॒रर्जु॑नं च॒ वि व॑र्तेते॒ रज॑सी वे॒द्याभि॑:।
जाय॑मानो॒ न राजावा॑तिर॒ज्ज्योति॑षा॒ग्निस्तमां॑सि॥ १॥
ahaśca kṛṣṇamahararjunaṃ ca vi vartete rajasī vedyābhiḥ
vaiśvānaro jāyamāno na rājāvātirajjyotiṣāgnistamāṃsi ॥1॥
The two beings (rajasī) rotate alternately (vi vartete), by means of these forces the dark period of the day (night) (ahaḥ kṛṣṇam) and the light period of the day (day) (ahaḥ arjunam) must be known (vedyābhiḥ). Fire (agniḥ), being born (jāyamānaḥ) within each person (vaiśvānaraḥ), like a king (na rājā), coming into being and overcoming (ava atirat) darkness (tamāṃsi) by light (jyotiṣā).
The text of the Rigveda does not directly say what kṛṣṇa (is dark day) and arjuna (is bright day), but I found clarification in Nirukta (2-21) by Yāska, he explains that these are the day and night – ‘ahaḥ ca kṛṣṇaṃ rātriḥ śuklaṃ ca ahaḥ arjunam’ (black and white days are also day and night). In this hymn of Vaiśvānara it is said that Vaiśvānara is beyond sleep and wakefulness, also Śankarācārya in his comments to this verse connects it with a state of consciousness, which is beyond sleep and wakefulness. We can also recall the famous metaphorical statement of the Gītā that when ordinary people sleep, the yogi is awake, and vice versa, e.g. the yogi focuses on dimension that is beyond normal human states. For the yogi, “the battle of Kṛṣṇa and Arjuna” is a process of victory over constantly changing impermanence during sleep and wakefulness. On the other hand, day and night are also forms of the Deities in the Vedas. For example, by falling asleep we give ourselves into the hands of Goddess Ratri, it returns us to our being and thereby restores the vital forces that are lost during the day. Sleep and awakening are a kind of “micro-rebirth,” in tantric yoga there are enough practices associated with sleep or dreams.
In general suṣumna is very glorifying in Haṭha-yoga-pradīpikā, many texts say that suṣumna contains the entire universe, it is very revered. The term itself comes from the root सुम्न (sumna), which means something desirable, magnificent with a strong prefix सु (su), where the “s” is transformed according to the sandhi rules into “ṣ”, thus the word suṣumna (very gorgeous) appeared. It comes up from the name that this channel is worthy of extremely serious perception, and Siddha-siddhānta-paddhati and a number of other texts describe it as a goal (lakṣya).
Sometimes there are translations of the term, like where the sun’s rays are. Obviously, this context comes from such early texts as Taittirīya Samhitā (188.8.131.52):
सुषुम्नः सूर्यरश्मिश्चन्द्रमा गन्धर्वस्तस्य नक्षत्राण्यप्सरसो वेकुरयः |
suṣumnaḥ sūryaraśmiścandramā gandharvastasya nakṣatrāṇyapsaraso vekurayaḥ |
Thanks to (suṣumna) moon, shining with the rays of the sun, is gāndharva, and his āpsaras (companions of gānharva) are the nakṣatras.
Further, it follows from the text, that this is described in the context of a fiery yajña, where the fire also relates to gānharva, the radiant rays of the flame – to āpsaras. In fact, the fire ofsuṣumna is the interiorised fire of the external yajña. Apparently, the term itself has Vedic origin.
Sometimes it is difficult to say where the object of reverence acts as a “support”, and where it is the goal.
A good example of the fact that when talking about tamas and relating it to Shiva, it is meant not simply one guna of the Prakriti, but something more, we can see in the earliest sources. Here is how it is said about the nature of darkness in “Nasadiya-suktam” (Rig Veda, Mandala 10. 129. 03):
तम आसीत्तमसा गूढमग्रेऽप्रकेतं सलिलं सर्वमा इदं ।
तुच्छ्येनाभ्वपिहितं यदासीत्तपसस्तन्महिना जायतैकं ॥३॥
tama āsīttamasā gūḍhamagre’praketaṃ salilaṃ sarvamā idaṃ ।
tucchyenābhvapihitaṃ yadāsīttapasastanmahinā jāyataikaṃ ॥3॥
At the very beginning of the creation of all existence was darkness, hidden by the very darkness, all these were waters. From a single tapas (the heat) in the void, the One was originated.
The darkness there is not just one of the qualities (as some of the Krishna’s followers interpret, for example) of the primary cause of the Prakriti creation, but indeed, this is the Great Abyss from which everything manifested itself and into which everything is absorbed back. Exactly in this context Shiva is also meant, when one speaks of His inherent darkness.