I want to dye my hair black with blue highlights. How do I go about that?

Do i have to beach it? If so for how long?I want to dye my hair black with blue highlights. How do I go about that?
If your hair is too dark you'll have to bleach it first; just bleach only the parts you are planning on dyeing blue and make sure not to leave the bleach on your hair for more than 1 hour. Then apply the blue dye to the bleached hair and cover it with foils and dye the rest of your hair black.