The fact that he has 3 more mouths on his body should make it a blood limit by itself, although the explosive style (earth+fire) could add to it but could also be categorized as a secret technique. And same goes for Hidan immortality.

Like you said, having multiple c*ra natures isnt a bloodline limit, look at Sarutobi and Kakuzu, they have all 5 natures and arent bloodline ninjas, they just mastered them individually. The bloodline is something that cant be coppied like onoki particle style (earth, wind and fire) or mokoton (water and earth) not even by a sharing*er (although a rinneg*er can do everything), you either are born with the ability to mix the elements or you're not.

Interesting thing is that the pains arent bloodline users althout they have the rinnegan, but they do have a symbol of their own, maybe because they are dead and only nagato will have the bloodline, though the eyes arent his but madara's.

Kisame is half man, half shark and he was born like that so it could also be a bloodline and later on the same goes for Fuguki, not to mention suigetsu and mangetsu that are basicly water.

its a fine line that even the manga/anime blurs from time to time.

they never explained its a pure mystery, but really freaky, same as hiddan immortality they only said its part of his religion (but how he became immortal?) and kakuzu with is strings inside the body what the hell is that.

and i totally forgot kidomaru and his four arms, also that other sound ninja with air tubes in his arms from the chunin exam