Hair Chalk - Is it any good for cosplay ?

Having dark blonde hair, I don't really fit into the Bat family so I tried a toner (non-permanent colourant). I couldn't find black, only darkbrown and it turned out as a wine-red when in bright light. It wouldn't even stick to my hair properly the first time I applied it.
I googled a bit and found a product called "hair chalk", the fancy folks in the beauty business call it the latest trend in hair coloring and they insist you can even get blonde hair black.

So has anyone ever used black hair chalk ? Is it any good for cons, especially when it gets a little sweaty ? And how much does it change you hair color ? Are we talking pitch black even though the base hair color is dark blonde or something similar ?

I don't have any personal experience with hair chalk, but a friend tried it for a Halloween party last year. It didn't stay in her hair too well, and she ended up with green (the color she used) everywhere. I don't think it would be too great at a con.

Mr Mold Maker said:

I don't have any personal experience with hair chalk, but a friend tried it for a Halloween party last year. It didn't stay in her hair too well, and she ended up with green (the color she used) everywhere. I don't think it would be too great at a con.

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Do you think hairspray might make the chalk stick to the hair ? The worst thing I can imagine is black sweat running down my neck and forehead, could that happen ?