I had umbilical hernia repair march 8. It is march 28 and I still have that black bruise above belly button. It hurt. Incision is in belly button, why?

Should see surgeon. A bruise should be improving by 20 days after surgery. You should see the surgeon if you haven't seen him or her recently so they can look at it. The black/blue/purple area could be the skin of and around the belly button that had its blood supply affected by the surgery. In rare cases, the surgeon may need a second procedure to revise this.
You will be OK. Some times small amount of blood under the skin causes bruise ( ecchymosis ) , make sure go back to your surgeon for follow up, and to make sure you have no infection.
Migrates. The hernia sac was likely removed and the tissue sewn together or a patch placed, depending on the defect size. In dissecting the area to sew some bruising occurs. Heating pad on low over the bruise can comfort and help resorb.