Incision from mastectomy 7 days post. Is it ok for white pus? & incision / skin is black. Necrosis? What happens later?

Must be SEEN. What you are describing can be as minor as a normal response to surgery or as serious as an infection requiring intervention. It is impossible to differentiate the two extremes by a simple description. I assure you--your surgeon will want to hear from you and see you as soon as possible.
Get followup ASAP. Your description suggests that the skin at the incision has a poor blood supply. White puss strongly suggests infection especially if the surrounding skin is red and you have been having fevers. Get in to see a doctor immeadiately.
No. White drainage is probably either lymph or drainage from mammary glands. Skin dark is tissue not healing well - will need wound consult. Tissue will heal slowly provided proper wound management procedures done.