I would probably first try adding some dark blue, because if the blue-black is now grey the blue has certainly deteriorated (probably also the black).
If you have some blue to spare it could be fun to try.
Follow advise and mix in a separate vial, and check that no precipitate forms. Also use small quantities initially, if it goes wrong throw the whole thing away...
Waiting for two weeks is possibly too long, Quink should be a rather tame ink, and if you mix it with some other western ink, like Quink itself, Pelikan, Montblanc, Aurora, JH, R&K, and similar it's unlikely to be very risky.
Try it, then leave it in the vial, and see if it remains liquid in the following days or forms a sediment at the bottom.
You can add black instead of blue, but if the starting ink is grey you'll get black... not blue black...
If adding blue does not give you a blue black...they you're probably better off just mixing some blue and some black from new bottles, or buy a ready made blue black
Edited by sansenri, 26 January 2020 - 20:18.