what does 군청색 대기 mean?? novel sentence

Question

첫 장면에 나타난 건, [[군청색 대기]], 그 가운데를 가로지르는 회색 번개였다.

here is the original sentence. I don't understand what the sentence means.

thanks in advance

Answer 1

군청색: ultramarine blue
대기: atmosphere

Do you understand the whole sentence except '군청색 대기'? If not, please reply to me.