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Dark brown or blackish blood indicates old oxidized blood. Sometimes it is considered to be normal in women who have delayed periods. If it occurs more often and apart from being associated with periods, then it could indicate a number of issues like: gonorrhea, genital warts, Chlamydia, menopausal changes, pelvic inflammatory diseases, HPV infection, and cervical cancer.
Go ahead with a thorough check done by your gynecologist (not GP) along with further investigations including a repeat PAP smear test to come to the exact cause to be able to start with treatment.
Take care.