I can well remember that there was a time when a deep blue sky.为什么SKY前要加“A“啊?

I can well remember that there was a time when a deep blue sky.为什么SKY前要加“A“啊?
原句是I can well remember that there was a time when a deep blue sky,the song of the birds.moonlight and flowers could never have kept me spellbound.为什么SKY前要加“A“啊?SKY难道成可数名词了?