Technically, yes, colouring hair is dangerous. Especially if you're going for unnatural colours.

BB has it right. For an unnatural colour to have maximum effect, hair must be bleached beforehand. This is extremely damaging. There's a reason why unnaturally coloured hair (blue, bright pink, etc. etc.) does not look healthy - it isn't!

The only answer to "fixing" bleached hair is either dying it back to its natural colour or just growing it out.

And yes, since your hair is dark, it would have to be bleached first in order for the blue to show. Actually, hair of any colour must be bleached first for an unnatural colour to work.

If you are trying to grow long hair, this process is not a good idea.