Question

the heavens themselves blaze forth the death of princes , "elucidate".

Answer 1

He decided to elucidate on the subject further. ... no comets seen, the heavens themselves blaze forth the death of princes”, he quoted.

Answer 2

here calpurnia (Caesar's wife) is trying to stop Caesar from leaving the house by telling him about the ominous dreams she had to emphasize her point she says "the heavens themselves blaze forth the death of princes"
which means that if any valuable person is about to die then all holy things try to point towards trough bad omen that is Caesar's death