A subtle hint that Judah would be the eventual representative of Israel, and not Reuben, Israel’s firstborn, is perhaps located in Gen 42.29-43.14. The sons of Israel must bring Benjamin to Joseph as Joseph has requested at their previous. Of course Jasob was terrified at hearing the very idea of Benjamin leaving his house, as Benjamin was the only son left from his most beloved wife, Rachel, since he thought Joseph was as good as died already.
Reuben was the first one to persuade Jacob to let Benjamin go: “Kill my two sons if I do not bring him back to you. Put him in my hands, and I will bring him back to you.” (Gen 42.37) Fair enough, since if Benjamin died, it means Jacob has lost two of his sons. So Reuben offered his own two sons as repayment. But the idea was rejected squarely by Jacob.
And next was Judah: “I will be a pledge of his safety. From my hand you shall require him. If I do not bring him back to you and set him before you, then let me bear the blame forever.” (Gen 43.9) There was indeed a difference here. Judah offered himself as a pledge and not his sons. But the point remained. Following Reuben, Judah was trying to persuade Jacob as well.
And he succeeded, indeed. Jacob eventually relented to the request of Judah and let Benjamin go. In some sense, then, Jacob’s blessings went to Judah, and not to Reuben, his biological firstborn.