When something is said to loom it is said to be more visible, prominent, distinguishable, noticeable. Easier to see. Easier to spot.
verb (used without object)
to appear indistinctly; come into view in indistinct and enlarged form:
The mountainous island loomed on the horizon.
to rise before the vision with an appearance of great or portentous size:
Suddenly a police officer loomed in front of him.
The word loom does not work to emphasize the above a, b statements you wrote. To loom out of the darkness simply means to appear out of the darkness; yes, you can replace 'out of' with 'appeared' in your example sentence. Common sense tells us that vehicles just don't pop out of the darkness. What the writer is then trying to do is exaggerate the appearance of the cars as the came out of the darkness, to show the reader the fact that the cars were speeding. This is known as a figure of speech. Specifically it is a hyperbole which is a type of metaphor
Let me further address what you wrote:
(a) The vehicles are not clear but still visible.
(b) The vehicles are visible but not clear.
I do understand what you are saying. But, let us read back "...out of the darkness..." this tells us that the vehicles appeared out of nothing. There is no need to focus on the visibility of the vehicles, we should be focusing upon the speed of the vehicles.
The vehicle was going so fast it is as if it just appeared out of the darkness at night. More literally, it was going so fast that you couldn't tell if it either just appeared out of nothing (impossible) or if it had actually driven to the location. The vehicle would presumably have headlights on.
Concluding, if you are trying to determine the visibility of the vehicles from this sentence, we assume the vehicle is fully visible.