It works as a black dye but...

When I started out with this dye, my hair was a very very pale shade of mint all over, as it was bleached before I over dyed it with teal. The teal had mostly faded so, as I say, it was pale mint, mostly back to white/blonde.
I over dyed it with the cosmic blue, hoping for it to come out a nice, fairly bright, blue, as shown on the box. It did for the most part too.

It's a dark navy under a fluorescent light, and under bright sunlight it has a blue shimmer to it, but under most light it just looks like a fairly rich shade of black.
I left the underside of my fringe undyed, (so as to redye it teal and have two-tone hair) and the dye has bled into the bleached portion leaving it a rather pleasing shade of blue/grey. I'm really rather hoping that it'll fade to a similar colour because black really doesn't suit me...

The dye itself was fairly cheap, and certainly there was a decent quantity of the stuff, I was left with around 1/3rd of the bottle unused, and my hair was thick with it. That said my hair is only just past chin length, although fairly thick by comparison to my friends.
The smell was similar to that of most other permanent dyes I've used, although most similar to hydrogen peroxide. It didn't particularly bother me, but it was rather strong, and if it's your first experience with dye, it wouldn't be particularly encouraging a smell, as it did make me feel a tad dizzy after a while.

The conditioner was lovely. I used it all at once, as well as my regular coconut oil one, alternating coconut, provided, coconut, provided, coconut, coconut. This is just my obsession with deep conditioning after dying it with a perma dye, but it left my hair wonderfully silky.

Overall I'd recommend this dye as a black one with a slightly blue tint, rather than a dark blue dye. It's lovely and rich and shimmery, but it's not a true dark blue.