Thanks for suggestions, so let me give you a little background on these guys to better understand the argument. The shadowdancer player is a min/max person and he is horrible at rping, while the other player is a flavor person and loves rping. So while I understand the ability description is left vague on purpose, I understand both sides and I'm having trouble making a judgement call. The fact these two are arguing isn't that uncommon. At this point I don't really care but with my players taking sides I might need to make a choice.
With such radically different players, a middle ground will be unsatisfactory at best. Based on the ability's other restrictions, I would allow him to count complete darkness as a shadow to jump around in.
For starters, using the full distance empties you for the day. Using it more than once requires making small jumps. As for complete darkness, Darkvision limits sight to 60 feet so any jump longer than his sight range risks him teleporting into an obstacle.