Genesis 27:30,31

‘And it happened, as soon as Isaac had made an end of blessing Jacob, and Jacob was yet scarce gone out from the presence of his father, that Esau his brother came in from his hunting. And he also made savoury meat, and brought it to his father.'

He left only just in time. Esau, confident of the benefits he is about to receive, arrives back at the camp and prepares the food for his father. Then he confidently strides into his father's tent. He is not too concerned about the fact that the blessing may counteract the oath he had made to Jacob. Once the blessing is given it cannot be taken away.

Genesis 27:31 b

‘And he said to his father, “Let my father arise and eat of his son' s venison, that your soul may bless me.'

Compare the similar words in verse 19. This was clearly the regular formula for opening the blessing procedures.