Above the city, the sky was at war; the last of the sun struck blood-red embers off [the] massed black cloud, while the clear horizon to the west was the colour of blue ice.
No, it's not gloomy.
I'd prefer a "the" where I have suggested it, but I know it's common to leave out determiners in popular fiction:
"She gazed into ice-blue eyes". I've never been a fan of it.
I keep reading "the sun-struck blood-red embers" and expecting a verb to follow.
Was that the question?