Black vaginal bleeding on implanon

Patient: I have been on the contraceptive implant for 7 months and had no bleeding since started. Before this I was on the depo shot but it caused me to bleed after sex so was changed to I plan on which solved this issue. For past 2 months I’ve had thick discharge and I mean reeeeally thick and now for past three weeks started bleeding. I am concerned because the blood is black. I have never seen blood this dark before and am worrying as it has gone on for three weeks with no change. There is no risk of a sti . thanks