The raspistill application has many parameters that I have yet to explore. Search online for examples. doc 
My Zero Webcam was producing solid black. I enhanced this with Preview's Auto Levels to get a ghostly speckled image with barely recognizable parts. I looked for suggestions online. reddit
raspistill -q 100 -t 1 -ss 6000000 -sh 0 -co 50 -br 50 -sa 0 -ev +2 -ex night -awb auto -ISO 800 -mm average -n -o dark.jpg
4:07 am, enlarge
4:43 am, enlarge
In this second exposure the faint blue light of dawn meets the yellow light from the neighbor's porch light and somehow comes out all color balanced. Amazing.