how long ago was it laid? because as they harden they do tend to darken overall, i think this takes place over a few hours or maybe a day i think. however if its been laid for a few weeks then i dont know whats happening. however, i don't think even if the mantids were dead it would turn black at the edges, i dont think youd see a noticeable difference. i could be wrong of course, im just making a (not very) educated guess.