And it came about, while Israel was living in that land, that Reuben went and slept with his father’s concubine Bilhah, and Israel heard about it.
Now there were twelve sons of Jacob - the sons of Leah were Reuben, Jacob’s firstborn, then Simeon, Levi, Judah, Issachar, and Zebulun; the sons of Rachel were Joseph and Benjamin; and the sons of Bilhah, Rachel’s female slave, were Dan and Naphtali; and the sons of Zilpah, Leah’s female slave, were Gad and Asher. These were the sons of Jacob who were born to him in Paddan-aram. Genesis 35:22-26 (NASB)
The text is calling out "firstborn" intentionally in Genesis 35. The text could have said "and slept with his mother's servant, Bilhah," because that's how Bilhah has been identified until now. Every single instance of "Bilhah" in Genesis associates her with Rachel. But here, it says "father's concubine" in order to paint a picture of a son standing against his own father.
Even later in the same chapter, Bilhah is again referred to as the servant of Rachel:
...the sons of Bilhah, Rachel’s maidservant, were Dan and Naphtali; Genesis 35:25 (NKJV)