This is an interesting problem. Tenebrae uno ante passu insidiantur is a pretty good translation; Insidiantur is an inspired choice, as it implies the darkness "lies in wait" just a short distance away.
However I'm not sure about passu; this word really means a "pace", and in Roman terms a pace was two steps (e.g. you start a new pace when you put down your left foot). We know this because the term for a Roman mile--fairly close to our own 5,280-foot mile from the markers that have been found along Roman roads--was mille passus - "a thousand paces". Not many folks even today (unless they play in the NBA) take five-foot long steps. So I'm inclined toward uno ante gradu here.
Ante paulum - "a little ahead" is another possibility (I personally don't like it). And if the OP wants to emphasize just how close the darkness is, I'd suggest dropping uno and adding vix - "barely, just" (this one I do like, but given the original English am not sure it's justified).
To summarize, I'd go with:
Tenebrae uno ante gradu insidiantur
and change the uno to vix if you prefer the interpretation "just ahead".