Hmm... well... they do seem a bit long. It could be normal, or it could be a sign of a problem. Dark thick blood/mucous is also common in most cases. It can vary from near black, bright red, to a rusty brown color. It has to do with the density and age of the blood. Fresh blood is bright red. Darker blood is slightly older and has likely been sitting in the uterine lining for a while. I would say that you should see your gynecologist to be on the safe side. Some times there can be complications with birth control and you may want to see about having the implant removed and trying a different method to better regulate your cycles. Good luck!

Let us know what you find out!