You can change your cross hair color by using the command cl_crosshaircolor in console.
Unfortunately, you can not change it to black using this command. cl_crosshaircolor 0 = green, 1 = red, 2 = blue, 3 = yellow, and 4 = skyblue.
I'm not sure if there is a way to change it to black, but it's not with the cl_crosshaircolor command.