Typically what I find when that dark sense comes up it is for a couple different main reasons. The first time I had that happen it was a sense of a dark, foreboding presence, sort of like the fear you feel if you watched a scary movie and are imaging scary things. Somehow intuitively I knew to not be afraid of this, that it was something from myself that I needed to instead embrace as a wounded child, to give it love instead of fear it. It was difficult at first, and I felt a certain sourness in my stomach. But the result was as I struggled with it that it's power was released and there was a rather powerful integration that happened, a certain healing of the dissociated, repressed, or suppressed aspect of myself. It was stuff from the subconscious you don't know is there, are not consciously aware of that manifests itself in symbolic ways. It comes from our culture, our personal self-dialogs, senses of rejection, etc.
To symbolically interact with it, is in very real sense seeing something you don't want to look at as an object, rather than the typical response of denial and suppression as a subjective experience. You in fact do not have to be able to know exactly what it was, as the thoughts about it are not always the core problem itself. It's usually far deeper than the "reasons" our mind want to look at, which is why it will come up as "the devil", or a black ooze, or some such fearful aspect. It's not the face, but the source, which is typically a wound, a self-rejection, a lack of self-acceptance, denial, repression and so forth.
That the first reason for fear in meditation, which is the subconscious trying to heal your conscious mind, make you whole so you can move forward. The second reason is growth into something new, by letting go of control, layer, by layer, by layer, room, by room by room, door by door. At each doorway stands an aspect of ourselves that is not ready to let go of itself to emerge into the new. We like security. We seek for answers. We seek for stability, knowing, safety, etc. We don't like change, to the point of say "all bets are off here", you aren't in control entering into what you don't know. In other words, we fear the unknown, that Void where we cease to exist as ourselves anymore, replaced by who we are becoming. Each time, that Light that is too great to bear overwhelms us with its presence, and we reflexively recoil as we feel ourselves losing ourselves into it. We create a "demon" at the door by telling ourselves it is something to fear. Our minds will bring up distracting thoughts of an emotional nature, "I'm not a good person. I did this or that", or "I will lose myself and come undone". We withdraw to protect ourselves, to feel safe in the "known". "Beyond this point lay dragons!", our ego says to us to protect itself from a perceived annihilation.
This is where "faith" comes in, I say. It is a trusting in letting go, of stepping off the edge of that cliff, not knowing what lays beyond, but knowing what is there is Release. The more we encounter this, the more we recognize it for what it is, the mind's defense to protect the ego sense. It's merely habit. Each time we learn to relax it and let it go, door by door, etc. But it still comes up. It's just of a different nature than the subconscious bringing up stuff about ourselves we don't want to look at. The former, the first fear is healing, it's psychotherapy (and I recommend if there is a lot there to maybe do this with the guidance of a counselor. The latter, the second fear is transformation, emergence, self-transcendence into who we are becoming, letting go of who we were, little mini deaths, again and again. I believe the first has to happen, before the second can.
Hope that's of some help me sharing my thoughts and experiences with this. BTW, I have found those who are fundamentalists will hit that fear, and call it the devil and dig even deeper holes to bury their psyches into in religious fundamentalism. These experience the least freedom in their religion, living in fear from beginning to end. They externalize the whole affair saying "Jesus will save me", and avoiding any inner work at all through this "true believer" of psychic escapism. I can go at some length more about this, but it's something useful to recognize.
You've learned to trust.
