I would say that in terms of an agentialism-as-existentialism (or here the French-American "New Existentialism,"), this means that agency, which is a common denominator to all things who struggle "to be" things - and we might also interpret agency to mean freedom, creativity, and persistence - must come first, and immanent condition later. For example in the case of "human" beings, being "human" only comes later, but the being (better, be-ing, or becoming) understood as agency, is first. Agentially, then, that activity transcends immanence through negation, and despite being within immanent conditions, through its freedom affords production and contribution to the universe. Thus, we are enabled to say that there is equal ground of agency among all free creators, whatever being, i.e. "thing" it is (this rings heavily of Sartre's appeal to Kant).
Given this, perhaps we may venture to say that the "darkness" referenced in the video is that abyss of freedom - the "pillar of freedom" Sartre mentions. And if it is not affected by, or tied to, immanent conditions, then nothing can touch it - so much so that not even one's own "self" can touch that be-ing of "self" which is first for there forever is no finished self to be. Put more accurately in an essay by Kitaro Nishida, "The bottom of my soul has such depth; Neither joy nor the waves of sorrow can reach it."
The New Existentialism appears to be a form of agentialism that returns to the "vital negative" present in existentialism minus any humanist or anthropocentric trappings. In other words, if it is possible, try to imagine existentialism as a philosophy where "the human" being has disappeared into an "ontological darkness." One might ask then, whence is "human" if it has disappeared into the dark of be-ing? (As a side note, see my post from many years ago, "Infinite Density and Aesthetics" HERE. I understand the religious aspect would turn off some After Nature readers; but many of the ideas under discussion here are there. So maybe in some sense this would lead in the direction of theistic existentialism for some; i.e. Kierkegaard.)
On a side note I was underwhelmed by Garcia's philosophical ability real-time. His boyish appropriation of a language obviously picked up from reading philosophy exclusively online is quite visible. He latches onto the language of branding by leaning on concepts such as "withdrawal" etc. etc. as well as ideas that come directly from blogs and blogs alone. (It's alright to start on blogs in order to gain a sense of direction or trajectory or to put one's finger on a pulse, but nothing can replace working through the texts in question themselves. *Dwelling* on online discussions - question without moving on to the arguments and texts in question themselves immediately challenges your credibility, especially real-time while at a conference where others are able to become immediately aware of whether or not your "philosophical chops" are real.)
[UPDATE March 2017: This occurred to me at a recent colloquium where I watched Jane Bennett give a talk. It was immediately visible she had no idea what she was talking about.]
In any case, watching Garcia talk I hardly had a sense that he is trained or even able as a philosopher. As sad as it is to say, in the end he just came off as an online groupie.
I say let his appearance and ability speak for itself. This way people won't be misled if he is peddled as the next greatest thing or flavor of the month.
These are harsh words I know, but I must speak the truth.