Based on experience, the rate is approximately 15%. Fleshformed and Powered Armor have already been confirmed as non-functional, suggesting that most, if not all, professional legendaries should be assumed to be incompatible. This also means naturals likely cannot have any bloodline.
Naturals *might* be able to get the Sorcery Legendaries at least, not sure about others.
From what I recall of H6 code, Sorcery doesn’t require being a supernatural. It merely requires the appropriate level of the arcane tree related, which I think requires being gifted.
Stats won’t be elligible if the benefit of the stat transcends the period you have it in. Like if someone has a bloodline artifact for sorcery then that’s a lot worse than actually having sorcery since you only can use it in fights every few days etc, if you had a bloodline artifact for fleshforming that’s basically exactly as useful as having the stat since you only use fleshforming every now and again and then the person stays fleshformed afterwards.