Yikes! Rose petals turn black on the edges for several reasons but none of them are fatal. This is caused by hot sun on a plant not used to warm weather, also from a cold shot of frosty air and thirdly from being hit with an herbicide or pesticide spray on a sunny day. You could just admire the interesting color combo because chances are the rose plant won't be showing the black tips again. Keep growing, Marianne Binetti