Self and Other
In the Absolute there is nothing, no-thing, no light or dark. When attempting to explain the experience of union with the Absolute, one might use terms like ‘nothingness’ and ‘darkness’, but this is not really accurate. There is nothing there, so nothingness works, but it also conveys other meanings that don’t work at all because everyone is there, and all potentialities are there (‘there’ being another inadequate term).
Tamas is a quality of nature, a guna, often translated as ‘darkness’, but the Absolute is beyond nature and its gunas. Also, tamas is characteristic of Rudra Loka and Raja Yoga. The idea of being ‘beyond tamas’ refers to being beyond Rudra Loka, and is not meant as a guna associated with the mind. Lord Krishna is using the term “beyond darkness” to indicate being beyond the gunas, and the completion of yoga sadhana.
Self and Other
You are Purusha. What you perceive is Prakriti.
Purusha is masculine gender, whereas what purusha is aware of (other purushas) is feminine gender, and called prakriti, thus expressing the theme of duality that makes up Creation (e.g, Adam and Eve).
You, a Divine Individual in the Absolute, are Purusha (Adhyatma with a viewpoint) perceiving another Divine Individual (Adhidaiva) as nature (Prakriti).
In the abstract, we speak of God as The Absolute, but in this verse (10) we are speaking of a Personal God. This is not to say that there are two Gods, but that there are two ways you can know Absolute God: as Personal (Adhidaiva, the first Divine Individual known to you in the Absolute) and Impersonal (Brahman, or the Absolute situation of all of us). God, being unlimited, can accommodate us in either case.