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.