The Blackfish was stunned. Of all the things, he did not expect this. After a few moments, he finally realised the reason for this request. He had the experience, and with his grandniece here, Winterfell would be secure in the absence of the king. Sansa had no backing and no way to usurp her brother as a woman, especially with her marriages to a Lannister and a Bolton. Brynden would also be able to have a greater hand in the defence and wellbeing of his grandniece, which was the reason he came here in the first place. If he swore in service to Jon Stark, he could do that and have a proper place in Winterfell.