Non permanent hair dye.

Okay I'd really like a straight up answer. My mum is confusing me, and trying to scare me out of dying my hair. lol
Basically, I'm using black non permanent hair dye that lasts 6-8 weeks on my natural kinda dirty blonde hair. On the pack it says it...

isn't recommended for light brown hair, why is that? Is it because it probably won't give me a really dark black I'm looking for?
Basically my question is:
If I use two hair dyes at once on my hair, will it actually make it black? Or because my hair is quite light, will it come out a dark brown?
I really want someone with experience in hairdressing here to help me out. No guesses please, I'm dying it tomorrow. thanks a lot :D

the sameee thingg happened to meee, dont useee 2 dyesss at onceee becauseit screwsss your hair up + it's not recomended for really light brown hair becauseee it wont color the way you nessisarly want it.

It's probably not recommended for light brown hair because from light brown to black is a big step and can be quite dramatic- If you use more than 2 hair dyes at once I doubt it will still come out as proper black. It's also not good for your hair if you use two dyes. Even though dyes colour, your natural hair colour will still determine the final shade.
My hair is also light brown and I dyed it non-permanenet 'mahongany'- it turned out bright orange!
The brightness of the colour will all depend on your shade of light brown...I really don't recommend using two dyes at the same time.
Even though it won't last that long, some of the colour will continue to stick.
But hey!, I was happy with the way my hair turned out and am planning to dye it 'mahogany' again soon.
But just start off using one dye at a time- black hairs quite a big leap!

Oh goodness hunny -
First off light brown going to black is not a big step, at all, c'mon it's black. It's just the strength of non-permanent isn't strong enough.

The reason why the box says that is because non-permanent hair dye isn't as strong as permanent hair dye, therefor when using it on light brown hair it's only going to put you at a dark brown. Using 2 boxes isn't going to make it darker.

The brand you're using simply isn't strong enough to take light brown all the way to black, this is because isn't non-permanent and they don't want everyone thinking they're going to get pitch black.

However this will put you at a dark, dark brown just not pitch black and that's why they don't recommend it because they don't want you getting the wrong results.

Using 2 boxes of hair dye should only be done if the person has really thick hair and cant cover their hair with just one box.

My conclusion:
Non-permanent black hair dye doesn't have the harsh chemicals that permanent hair dye does so that's why it will just go dark brown and not fully black.

I think you might like the dark brown though - I was born with dirty-blonde hair also and I think dark brown looks much better than pitch black