Hi,
Based on it description it seems like some kind of MINOR BLEEDING due to an SIDE EFFECT from the MEDICATION (BLOOD THINNER) for IRREGULAR HEART RHYTHM.
let me explain,
You are long term patient, for
high blood pressure , seizures and irregular heart rhythm (
arrhythmia), now the doctors mostly tends to prescribe the medication which they call it BLOOD THINNER in every day terms which is very important to prevents
heart attack as well as
brain damage (
shock) due to some blood clot getting lodge into an important supply leading to cut off the blood supply to that organ and hence, if its cut of for heart we call it heart attack.
Now the black discharge is the blood clotted which may be leaking from some where in your private area or the anus, this is a common side effect for the blood thinner kind of medication , since I don't have the list of medication you take I can't pinpoint the medication causing that problem.
ADVICE:
CONSULT PHYSICIAN - talk to them about the issues facing and please take the list/ medication you are taking so that they can pinpoint the cause and help you.
Here most of the physician tend to do.
1) Physical exam , and will inspect the area from which black discharge is coming from.
2) some
blood test to check the drug level / clotting time for blood
3) may need some other diagnostic test to locate the source and try to fix it.
4) may reduce the dosage of
blood thinning medication or may change to other safer option.
Hope it helps.