On a calibrated monitor, assuming you mean for it to be viewed in the sRGB IEC61966-2.1 profile, it appears a VERY dark blue, nearly black. I suggest your "Windows machines", if showing something other than "almost black", may be uncalibrated and showing you inaccurate color.
OH man, that...is...CRAZYYYYYY! I had to open both the image on black and the orriginal on my ipad in diferent tabs... and the one on white still loked pure black... while the one on black looked more blue... to insure that I was not seeing an optical illusion I took 5 or six sheets of paper and cut a hole in the middle so only the hand would see when placed over the ipad as I switched between tabs... sure enough, it was an optical illusion! I amso glad you helped me with this before I sent this off to the printer!