By too dark do you mean that the back couldn't be seen at all and that it looks all black instead of translucent? Any chance that you have a picture of the Wisp on your black iPhone?
I'm in a dilemma between choosing the Wisp or the Frost for my phone, which is not the 5s Space Grey but rather the old iPhone 5 Black. So I assume that the Frost would look ugly on it since it would totally flush the colour out, like the Thinshield for example:
http://i.imgur.com/hDa6Tc2.png
Based on some photos I saw here, the Space Grey 5s look great in both Frost and Wisp because of the noticeable two tone back. Not sure if I can say the same with the Black iPhone. I guesss I'm leaning more towards the Wisp but I still want the back to be seen
Ugh I'm so confused lol. The pain of being a non-5s user