I have a bump under my breast and it started to turn black what could that be ?

Breast lump: hard to know exactly what it is, the best thing to do is to go see a doctor for evaluation who may order a mammogram/breast sonogram

Several Things: First of all, any new lump or bump under or on the breast is best evaluated in-person by a physician. In your case, the change in color could be related to a superficial skin issue (like a bump or bruise) vs infection (such as that from an insect bite or other) vs something more sinister such as breast cancer. Again, suggest an in-person evaluation.