Fourth line is missing a syllable, but that's nothing big. My biggest issue with this has to be that the second line is meaningless. "Beautiful dark sky" and "lovely and menacing" both give the same image. In such a concise form of poetry, two lines and fourteen syllables longer than a haiku, reiterating the point of the first line doesn't so so well. The third and fourth line, too. "Bright flashes of light" and "lightning" are equivalent. Nevertheless, the last line punctuates this piece very well.
Beautiful dark sky, So beautiful and dark, Lightning, Deadly thunder, lightning. When will the blue sky come back?
The last line works. I don't see the second and last half of the fourth working so well. There isn't a need to say it again, but maybe having both bright flashes of light and lightning might work, although not for your reason. There isn't much difference to me if it's labelled deadly or not. It's assumed to be deadly. It might work for something that meant "Lighting flashes. Thunder. More lightning." which is how a thunderstorm sometimes works. And then the implied Thunder.
It works, the first and fourth lines, or maybe the first third, and first half of the fourth line. If it read something like
.... sky lovely and menacing bright flashes of light deadly thunder .... When will the blue sky come back?
or
Beautiful dark sky, .... Bright flashes of light, Deadly thunder, .... When will the blue sky come back?
If the .... are different words that highlight different events and imagery of the storm, perhaps the rain or wind, this would be excellent. If it's beautiful it's already implied that it's lovely, so there's no need to say it again. If it's dark, it can be implied that it is menacing, and in context, it is. I'm just not that into how the first two lines produce the same image.
But in it's current wording, it would work better if one comma (or all of the ones at the end of the line) was removed (or the "so" in that line) so that it would play off the first line and say that the bright flashes of light were also lovely and menacing.