I just watched the final episode as well. The blood thing isn't that simple, you see the blood is useless, until the morphic field comes to play and that thing through the center of earth, even though it wasn't well explained then again what really was explained thoroughly in Torchwood compared to Doctor Who, very few things, most were just mysteries because of course they can't have clear answers that Doctor can have.
But...the blood on itself is part of Jack, it's part of the fixed point somehow I presume. But the whole point is that they fed his blood to the thing and it made the morphic field in its "mirror", but not inherited the real fixed point properties of course, I mean they were just undying, not regenerating etc. I agree, there should have been more explanation to this and it is shoddy writing and it fell flat in the end despite having great moments. The ending was kind of letdown too, it got rather really epic and then it slowed down and the actual ending was left with sort of cliffhanger in the end, so not really happy with that.