I'm a permanent wearer of all black. Tomboy style in all black is easily achievable in ripped black jeans and dark oversized jumpers and flannels (e. g. dark red flannel shirt over a black top. Paired with black jeans (ripped or otherwise) with your choice of boots or converse or whatever) Alternative fashion is a great place to find inspiration from. As is searching terms such as 'grunge fashion' or 'alternative fashion' on sites like tumblr and weheartit. Even guys style would be a great place to get ideas from since statement sweatshirts and jeans are easily found in black for colder climates. Jumpers that come past your wrists are considered cute and if the jumper is long enough it could be worn with leggings. In hotter climates 'skater girl' or 'skater guy' fashion could be copied using all black items (or mostly black items) and thin materials.