This seems most likely- that the blights and the heart are akin but are different manifestations of the darkness power. The blights are characterized as 'tears' in our reality and open into a different dimension possibly a reality dominated by the darkness. The taken are living things that enter these tears and are changed, modified and infused with power. I think that it is the darkness itself that changes and infuses these creatures and oryx/Malock merely create the tears. The taken spew from these blights and we have witnessed blights abducting cabal on phobos. In the black garden something different happened, the heart itself infused the sol progeny with power and was itself diminished after each infusion. I would venture that the heart was a vestige of darkness rather than a traditional blight. The grimior tells of an epic battle between a powerful entity and the darkness in the black garden and the darkness held the mastery. At first I believed this was rasputin but I think perhaps it was an agent of the vex that did battle with the darkness and either wounded it and worships this fragment of the greater whole or it is a sentry left by the darkness to supervise and guide the vex.