I'm going to discuss the ontology of being, briefly with Sartre's ideas of being. 
According to Sartre, there are 3 types of being: being-in-itself (être en soi), being-for-itself (être pour soi) et being-for-others (être pour autrui). 
Being-in-itself is simply being. It has no consciousness, therefore it has no potentiality for transcendence. This mode of being mostly applies to objects, like a chair or a stone. Being-in-itself is similar to self-negation or nililism as a things is just in the state of being. 
Sartre puts 3 characterizations for it: being is; being is what it is and being is in itself. "Being is" means being is not "derived from possibility or reduced to necessity". "Being is what it is" in the way that it doesn't need to refer to anything external to give itself meaning. Everything about it is intrinsic to itself and it does not obtain its nature through the relation with others. "Being is in-itself" means that it is not dependent on any construction to exist and affirm its existence. It doesn't need to be thought of, or to align with any construction to exist. 
The second type of being is being-for-itself, which primarily states that we have no absolute, fixed or eternal nature. We are what we make ourselves to be. We could reach for transcendence as we are no longer a being-in-itself. To Sartre, this phrase describes the state of human beings perfectly as humans possess the ability to transcend himself beyond what they are. 
Being for itself is the being of consciousness, for example, the being of humans. Yet, being for itself is the opposite of being in itself. Being for itself is not being what it is, but being what it is not. The being and essence of consciousness are not to be found in the act of contemplating, nor the object itself, but rather in the relationship with external world. Being for itself is not internal to itself, nor intrinsic to itself. Consciousness is pure directedness and intentionality.
From this point, one can claim that consciousness is nothingness.