A bloodline, that was possessed by a dragon can only be passed on to its children. It was the same with every creature. Unless there is a possibility of crossbreeding, the bloodline can't be passed to other creatures.
To make it possible, the other creature has to kill and steal the bloodline of the higher-level creature, which is not an easy thing.
There was a reason why bloodlines played a major role in deciding hierarchy among creatures. The bloodline gives unfair advantages to the creatures. Even a creature with a low rank can defeat a high-ranked creature, as long as its bloodline is high and pure.
When a creature obtains a bloodline, either through birth or by looting, the purity and grade wouldn't always be the same as the creature they inherited from. The birthed creatures have better luck with this as the bloodline purity and grade depend on its parents.