Can a character that has the Harbinger of the Abyss flaw be used as the source for Arms of Ahriman? Could a Talon of the Abyss shadow elemental be used as the source for Arms of Ahriman?
There appears to be a distinction between "shadow" and "darkness."
Arms of Ahriman can emerge from an area of "shadow" — but "shadow" isn't the stuff of which Obtenebration is made. Obtenebration is made of "darkness," which has some tangible quality and is able to do things like put out fires and inflict harm. If you're doing something that manipulates shadow, like Shadow Play or summoning elementals with Eyes of the Abyss, you can launch a tentacle from there. However, the Talons and the Harbinger flaw produce Obtenebration "dark matter," which can't be necessarily be used as a tentacle spawn spot.
There is a huge difference in WOD between shadow and darkness. The darkness a Lasombra controls comes from deep within; the shadow from a land called the abyss. Legend has that the Lasombra antediluvian communed with this being of darkness on the bottom of the oceans of the spirit world...
So my answer would be Harbinger of the Abyss, no; Talon of the Abyss can, by the rules. Although in terms of mecanics, in my humble view it would only suport one singular arm.