For this I actually use the total darkness rule from Unisystem
Use a D10 roll with no other modifiers; only a natural roll of 9 or higher strikes the target. If a character makes a Difficult Perception Test, she can add each Success Level to the D10 roll, accounting for the use of senses other than sight to spot the target for this test you can only add the Increased hearing modifier.
Author: Federico
I am the creator of this blog and a dreamer that converts it’s dreams into games.