Dark Release is a Kekkei Genkai said to be originated from Kirigakure, although by an unknown shinobi from it. Hiruko the villain stole this ninja's Dark Release KG and showcased numerous abilities with it. Dark works in absorbing other energy sources and redirecting them in a far stronger version of raw energy. The colour of the released chakra is light blue, and not almost black in colour.
The method I've used in order to make a better understanding of Dark will make things less confusing for you, at least I hope so. Dark Release is divided into two categories, that of "Absorption" based Dark and "Release" based dark. Now the former deals in absorbing chakra (mostly energy based elements, however higher ranked Dark techniques can even absorb Water or Earth at some point). The dark marks located in your palm need to be short range away from an attack for them to absorb it most of the time. Whereas the Released version of Dark is releasing the absorbed chakra in the form of raw energy, which is alike to light blue flames. This form of Dark does not have the ability to absorb things. Normally, a hit from this raw energy grants death to almost anyone. A B-rank dark explosion is enough to deal life-threatening damage. Another branch of the Released version is that beside the blue flames, it can also come in blast and shockwave version; things which will be portrayed to you during the training.
Moving on, you're right on the part that the absorbed chakra does not affect our chakra system or body, as they stay within the marks. It is believed that the marks are portals to other dimensions, (Example: Obito's Kamui) and these dimensions can contain an infinite amount of chakra in there. That said, let me ask you a question; can Dark Release absorb Senjutsu? No, or yes, provide me the reasons.
Lastly, in the RP, Dark is composed of Lightning and Fire chakra.