For non fire you can use normal climbing rope, the thinner the better, but not thicker than your pinky finger. I never had problems with blisters and by now the skin on my hands is hard enough to use pretty much any rope and not get burned. Also, climbing rope gets softer with use.
For fire dart I would do as LiveWire suggests (I never burned fire before so Im only guessing). A chain (as thick as the rope you use) between the rope and the dart would do the trick, about half a meter to one meter maybe? I heard kevlar ropes arent the best for rope darts, they are to rough.
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Aliens abducted me! They made me wear a funny hat.