1) We need more information to help you. What are your clothes made of? Cotton? Polyester? Rayon? Wool? A blend? There are different types of dyes meant for different types of fabrics, and these dyes are used in different ways. If you use the wrong type of dye, the color won't take, it won't come out right, or the process may damage your clothes.
2) If you want to go black, you'll need to use a higher quantity of dye than you normally would for other colors. Black is very intense so you have to saturate the hell out of the fabric. (Also, not all dyes are made equal.)
3) There are different types of blacks: red based, green based, blue based, etc. This under-color will show through to some extent.
4) If the clothes you start with are different colors (say, white shirt and blue pants), they will NOT dye to the exact same matching black. If you can get them both dark enough, the difference may end up being really subtle and not a big deal. On the other hand, it may be very noticeable if you can't get the color to saturate enough. Your final outcome will not look like a matched uniform, if that's what you're hoping for.
FF9 cosplay? Hit me up!
I now have a Tumblr
. Sometimes I use it.