At first I wanted to explain it by myself but why trying and failing if you got the internet? ^^
I think that in order to create our own Black Legion, we have to understand what the real Black Legion actually is.
Here are some quotes from warhammer wiki, which themselves are quotes from the Talon of Horus novel:
We were born for battle, Khayon. We were made to conquer the galaxy, not to rot in Hell and die upon our brothers' blades. Who are the architects of the Imperium? Who fought to purge its territory of aliens and expand its borders? Who brought rebellious worlds to heel and slaughtered those who refused the light of progress? Who walked from one side of the galaxy to the other, marking their passage in a trail of treacherous dead? This is our Imperium. Built across the worlds we burned, over bones we broke, with the blood we shed. You see it too. You feel it now, don't you? - A new war. One not born of bitterness nor founded on revenge. - The Long War, Khayon.
Abaddon sharing his vision
Sharing his vision with the assembled Legionaries, Abaddon knew that they all could become so much more than their fathers' sons. Both he, and those Legionaries before him, all craved true, honest brotherhood. They all missed it -- a Space Marine Legion's unity and its bonds of loyalty. Its explicit purpose. Its focused pursuit of victory. Abaddon missed what a Legion could do, and the fact it was empowered to do it. All of the Nine Traitor Legions, they were Legions in name, colour and the dregs of culture, but they were a horde, not an army, linked by fading loyalties and fighting to survive. Once they were bound by brotherhood and fought only to win. Their kind no longer waged war, they raided and pillaged. They no longer marched in regiments and battalions, but scattered in packs and warbands. Abaddon had no wish to change how things were -- he wished to embrace it. He knew that many amongst the Nine Traitor Legions cried out to be part of a true Legion once more. Abaddon's pilgrimage with Sargon had been more than learning how the tides of the Eye of Terror ebbed and flowed. It was about seeking those who would stand with him.
He meant to form a new Legion. A new war. The real war, the Long War. Not a petty rebellion swallowed by Horus' pride and his hunger for the Terran Throne. A war for the future of Mankind. Horus would have sold out Humanity to the Pantheon of Chaos for the chance to sit on the Golden Throne for a single heartbeat. But the Nine Traitor Legions could now allow themselves to be used the way Horus had been. The Chaos Gods existed and they could not pretend otherwise, nor could they allow a sacred duty to devolve into such weakness, as Horus did.
As you can see, the Black Legion had broken every single tie with their former parent legions. It is a bond of brothers, fighting to conquer the Galaxy and not for the needs of gods and failed Primarchs.
That is why they founded a new Legion, the Black Legion.
And that's why I think that our Black Legion needs a different name. From what I've read from blunt about Athrawes, he thinks in the same way like Abaddon did concerning his father and the failed Primarchs. Taking up the name of Harbringers doesn't feel right to me for the quoted reasons.
Edited by Kelborn, 30 October 2016 - 10:22 PM.