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Steven,
It is ideal to have grey flats, but, it's not a problem if you have a blue tinge, as they're treated as greyscale images in pre-processing.
The blue cast you're getting may be due to incorrect white balance settings in your camera. See if you can set it to auto white balance.
Mitchell, re: your question about the maths behind flat frames: darks are subtracted from your lights; and your lights are divided by your flat fields.
Regards,
Humayun
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