I have recently started DSLR photography, and love taking long exposures of the night sky.
I would like to have the sky be black, instead of blue, seen here: https://flic.kr/p/xRVXcw
I have a Nikon D5000, and would like to know what settings I should use for a black sky.
Answer
As WayneF commented in your question, it looks like you took the picture too early in the evening. The EXIF data indicates you took it at 8:49 PM DST in late August. You are probably in the period called astronomical twilight, defined as when the sun is between 12˚ and 18˚ below the horizon. Wait a bit longer, and the sky should be as dark as it's going to get for your location.
If the moon is in the sky, even a sliver, there will be enough light to turn the sky blue. The sky is blue due to a phenomenon called Rayleigh scattering, which is the diffuse scattering of sunlight. The light from the moon is just reflected sunlight, albeit about a million times weaker in intensity (or about 18-20 stops in photographic terms). Based on the EXIF date and time of your photo, the moon could have been up for as much as 3 hours in, say, New England, for example. In fact, the 27th of August was 2 days before the full moon, and it was about 96% of fully illuminated.
But without even consulting moon tables/calculators, you can tell there is some significant diffuse light in your photo because of the general daylight-like quality of lighting and highlights in the trees.
Note that depending on exactly where you are shooting, there might be too much light pollution to ever get a truly "dark sky".
There are several resources to find your location's sunset/sunrise, moonset/moonrise, and civil/nautical/astronomical twilight times, such as timeanddate.com on the web, The Photographer's Ephemeris for mobile and desktop apps, to name a couple.
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