Blood Release is an advanced nature Kekkei Genkai, but is quite different from most other advanced nature Kekkei Genkai. Most Kekkei Genkai fall into one of two categories. The first category is combining two elements together to ceate another elemental jutsu, such as with Wood Release and Ice Release, among others. The other would be like Kimmimaru Kaguya's Shitotsumyaku (屍骨脈; Literally meaning "Dead Bone Pulse", Meaning (Viz) "Macabre Bone Pulse") which is a rare technique passed down within an individual clan, but has nothing to do with nature releases. Blood Release, however, simply put, is just Water Release. Or more specifically, it's an advanced form of Water Release, which onlly those who have completely mastered Water Release and extended their power beyond the presipice of that Jutsu are able to use. With that logic, however, technically speaking anyone is potentially capable of using Blood Release, so long as they, at least for a time, focus primarily, if not only, on enhancing their Grandmastery of Water Release. Because of this, even though the potential does exist for any Water Release user to unlock this Kekkei Genkai, it's still one of the rarest Kekkei Genkai out there, and thus it's not seen very often at all.