I am working on a cake like the 2nd one shown on the link above. Should I use black gel or black royal icing for the scroll work? Would either run in the fridge? I do place my fondant cakes in a box in the fridge with great success but not sure what to expect with black scroll work.
Running will happen if the cake gets exposed to condensation, which is more likely to happen if you pull a cold cake into a warm humid room. If you wrap the box in plastic wrap really well, and let the cake come to temperature for a few hours before opening, you'd reduce the risk greatly.
That being said, I think both are a little prone to bleeding on fondant. I would do a test strip if you have time and see what happens with either coming in and out of the fridge.
It's a different effect, but I would paint the scroll work onto the cake with vodka/food color or use edible writers. Hope that helps!