"Face of Boe?? Looks like Rex! And it is an RTD character in a RTD world..."
No, no, no, no, again I say no. Ugh.
Okay, I get why Rex is now immortal. It wasn't Jack's blood that did it- it was the fact that he had Jack's blood in him, and the Blessing "thought" he shouldn't be mortal and "fixed" him. That does kinda make sense, because Jack's blood really isn't supposed to be special anyway!