What Causes Dark Menstrual Bleeding?

for the past few menstrual cycles, I have had dark red blood (almost black). The last 2 days or so, it would be normal blood and caused no concern. during my February cycle it was ONLY black and it had a tar-like texture. It came at the normal time, so I (tried) didn t worry about it. I had some spotting for a couple of days about a week later, but I have missed my normal period time now, and I last had sex 5 weeks ago. Is it possible I m pregnant. I took a home test a couple of days ago and it was negative. My doctor won t be able to see me for 3 more weeks. I stopped using the nuvaring after about 8 years, once on nexplanon, and then back to nuvaring for about a year. I have been off bc for the past 4-5 months and that is when I had the change in my period blood.