If I had wanted to be rude, I wouldn't have said please.
I don't usually go out of my way to tell people to look in other threads, but I
just answered this same question yesterday. You'd be surprised how many detailed responses there are, even if the person asking the question just asks a one-liner like, "OMGWTF HOW DO I GET MY HAIR BLACK FOR ONE DAY ONLY WITHOUT A WIG OR THAT SPRAY-ON CRAP?!!?" For example:
"If your hair is brown and you try to dye it black, the dye WILL stain your hair, even though it's supposed to wash out in 8-12 shampoos. I discovered this the hard way. I have naturally light brown hair but I dyed it dark brown. The dye was supposed to come out in a month, but it remained dark brown for about two months and then faded to a medium brown. About 5 months later, my hair is close to my natural color, but it's still on the dark side, and is more like my winter coloring."
"In short, temporary black hair dye is NOT temporary. No matter what the box it comes in says, no matter how convincing the argument is, no matter what ANYONE says--temporary black hair dye is permanent.
So, you can either dye away at your own risk, or you can get a wig. It's true that long black wigs don't exactly come cheap, but if you keep your eyes peeled (and there are NUMEROUS threads here on c.com that LIST several reputable online wig stores), you might find one that falls in your budget. Also, black-haired anime characters are a dime-a-dozen, and chances are pretty good that if you watch a LOT of anime, and you're a hardcore cosplayer, you'll find more than one use for that wig. A black wig is like a little black dress--it's ALWAYS good to have in your closet."
Those are two very detailed responses that probably answer your question, and those are from the links I provided you. Maybe next time you'll believe people when they say, "Use the search engine! It REALLY works!"