Detail? You know me.
Dark Release is a rare and advanced bloodline limit possessed by Kirigakure ninjas. It isn't your normal advanced combination of elemental releases like Storm, Boil, Scorch, etc. It is basically used to absorb the energy or chakra from the opponent's technique, manipulate the said chakra and release it in the form of raw chakra, albeit much stronger and denser than raw chakra. Those who possess this bloodline limit have two marks on their hands, resembling squares one below the other. The upper square is used to absorb chakra and store it in an entirely different plane in the user's body, i.e. this doesn't mix with the user's chakra network. The advantages of this is that even potentially dangerous chakra like Sennin chakra can be absorbed and stored. It can be said that these marks act like portals. The absorbed chakra is then manipulated and released in the form of raw energy blasts from the lower mark. Only raw energy can be released since the user has little control over the absorbed chakra, as it is stored in an entirely different plane, i.e. if a Fire Release technique is absorbed, then the user cannot manipulate the absorbed Fire chakra to convert it into a specific elemental one, rather it can only be released as a raw and dense chakra blast as mentioned before. It can be said that this is a psuedo-Fuuinjutsu, since it almost operates on the same principle of "storing" chakra.
Dark Release is a very interesting Kekkai Genkai. It isn't your usual run-off-the-mill "advanced fire" or "advanced lightning" etc. A normal advanced release would obviously have some trouble over Sennin chakra and Bijuu chakra. However, this can simply absorb chakra, manipulate it and release it. Even if the user is in a tight spot when a high level technique is used, he can simply do the above and put the opponent in danger instead, obviously following the elemental weaknesses and strengths. My bio, Portgas Ace would have Rain element if I pass the test, and I'd like to believe that this KG would be an excellent combo with Rain. The CJ combos are endless. That is why I'd like to learn Dark Release.
Although, canonically not known, in our RP, Fire and Lightning elements entail Dark Release.
Weakness and strengths are divided into two parts:
Absorption :
Strong to: Raw chakra, Fire, Lightning, Wind, Storm, Scorch, Sennin chakra. The basis of Dark Release's absorption is it's magnificence of absorbing raw energy and elements which have a low density. Raw chakra specifically, since C-Rank Dark can absorb A-Rank of raw chakra.
Weak to: Water and Earth. Earth is a very solid state of matter, and Water has a great amount of tangibility and Suiton jutsus are basically matter based techniques. That is why it is hard to absorb such highly dense elements. This is primarily why, S-Rank Dark is required to absorb B-Rank Earth.
Release:
Strong to: Wind and Fire. They are the least dense elements out of the basic 5 and they don't have the strength to overcome the raw energy blasts released by Dark Release. Elemental chakra> Raw chakra, but Dark chakra is much more denser than raw chakra, overpowering Wind and Fire. Similarly, Snow, Rain, etc have a low tangibility, that is they aren't exactly perceptible by touch.
Weak to: Earth and Water. Earth and Water have the solid power and tangibility to actually overpower raw energy blasts released by the lower mark. Higher solid forms like Steel, Wood, etc have a great strength to raw energy blasts, obviously because they're more tangible and solid.
Hiruko is the only canon user to be shown using Dark Release. However, he stole it from another ninja using his Chimera technique, meaning that specific ninja's clan may have also possessed this release.