The gas itself doesn't provide propulsion, instead it drives mechanisms that aim, shoot, and reel in the grapples. The gas you see coming out the back is waste gas being exhausted.
"Black box" in this case is just being used to refer to a mechanism where the implementation details are unknown, irrelevant, or, in this case, secret: <a href="https://en.wikipedia.org/wiki/Black_box" rel="nofollow noopener" target="_blank">https://en.wikipedia.org/wiki/Black_box</a>