I voted dark blue because that's what I think it is based on all the various descriptions - but I would also call the colors used for the LN's covers 'dark blue' (or 'blue' at worst, but definitely not 'really light blue').
So because I have too much free time, I made a (poor) infographic of the layers of her hair shades across the LN (P1V1 cover), manga (P1V1 cover), and anime (S2 wiki sample image), using the top Google Images result of 紺色 as the backdrop color for comparison. It's not a perfect representation, because even just googling a color gives you different shades, and different displays are going to present colors slightly differently, and you could argue about which exact selections to pick from the images as representative colors, but it is what it is.
Myne's Hair Infographic
For the LN and manga, (1) is the darkest shadow, (2) is slight shadow/darker neutral, (3) is lighter neutral and (4) is the highlight. The anime only really used three colors so I just did (1) for shadow, (2) for neutral, and (3) for highlight.
I think this makes it visible that the LN has the darkest hair color on her, with its (2) still being darker than 紺色 unlike the others, and although its (3) is somewhat lighter, it's not by much. The manga and especially the anime creep into lighter tones of blue, but I wouldn't say the LN's covers are too far off the mark for a 'dark blue' shade. It's not the darkest of dark blues or a midnight sky, but I personally would call it acceptably dark.