“It was a warm night, and through the open door you saw countless stars in a heaven that was still almost blue.”–HONOLULU
I wonder whether the word “heaven” is a countable noun. If the noun is countable grammatically, how many heavens have you seen in your life?
Should “in a heaven” read “in THE heaven” like “under THE sun” or “in THE sky”?
No, we would only say “the” with the plural (e.g., “the heavens above”). Normally, you would hear the singular in a religious context and the plural when referring to the sky. The example you give is a more flowery style of writing that you’ll hear from time to time.
Thank you for your explanation.
“Heaven” is the name of a place, real or imagined, usually assumed to be in the general area above people. “The heavens” is a synonym for “the sky”; I am not sure why it is pluralised. Using “a” in front of “sky”, “sun”, or “heaven” is an interpretation of these things as having a completely different character depending on when you are experiencing them , therefore each experience is one of an infinite set.
How many skies do we have? Do we count skies?! 
In this context, there is nothing wrong with saying “in a heaven that was almost blue”.
We can say “in A red sky” for example, emphasizing the colour of the sky in that vicinity. One might see a red sky, but the sky wouldn’t necessarily be red everywhere else.
Similarly, we can say AN expanse, or A heaven.
The “heaven” in the above context was a section/expanse of night sky in their vicinity, that still looked almost blue. (Away from city lights or other interfering light sources, night sky illuminated by moonlight is blue, similar to the sunlit daytime sky).
By the way…we can also say “under A warm sun”…
This might all be British English, but I’m pretty sure that I’ve read the same thing before in American literature.
I’m in 7th heaven
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Are you in the Seventh Heaven, the seventh heaven, or seventh heaven?
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I’m in seventh heaven, I arrived here after floating on cloud nine.
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