Yes, I think it’s a rare word even to see it. I have never used it by myself, though it’s not a slung. I checked with(on?) three web dictionaries and three paper Japanese-Japanese dictionaries I have. I found all of the web ones had it, but only the largest(?) paper dictionary had it. Unfortunately any of those didn’t have its origin but a same example sentence quoted from a novel written about 120 years ago. And you found it as Chinese, it pretty much seems like it was a loan word from Chinese. How do you think? I guess the musician likes the word more than common 真夜中(まよなか/midnight), or wanted to get rid of a cliche from the title, perhaps? We have many songs which titles start with the word 真夜中 or ミッドナイト( loaned from midnight). I hope this would make sense. By the way, why do you think it’s odd that the pronunciation of 午夜 consists of only ON readings? I think a noun written in more than two kanjis usually consists of only ON readings or only KUN readings. It is less common to find a combination of ON and KUN readings in one word.