Blake says they couldn't stand in his way in moving to City.
He said, “Nathan has been an integral part of the club's success in recent years and I cannot speak highly enough of him.
“Not only has he been outstanding on the pitch for us, but he has also been a fantastic servant and a model professional.
“Losing your best players is never easy but Nathan fully deserves this move and he leaves with our best wishes. We are delighted that this will give him the opportunity to showcase his talents at the very highest level."