I noticed a black triangle come in on one tooth, plus i have braces. is it caused by braces?

Dark Triangle: The dark triangles between your teeth often appear during orthodontic treatment. When teeth are overlapped at the start of treatment the area between the teeth probably doesn't have a normally formed gingival papillae. One the teeth are aligned, the lack of a normal papillae become evident. Several possible treatments exist including reshaping the teeth and cosmetic bonding of the teeth.

Could be bone loss: I am not quite certain what you mean. If you mean between two teeth at the gum level, it may be due to bone loss which in turn leads to the loss of the triangle of tissue between teeth. This appears as a 'black triangle' but in reality is a space rather than being filled by gum tissue. Often food gets stuck in this area as well. Have your orthodontist and your dentist check this area soon.