Dark period does tend to occur and is generally normal.
It could simply mean some old trapped blood of he last cycle.
If associated with odor,then it could mean some infection inside and must be checked in that case.
If there is no pain or no odor,then you may simply overlook it this time and see to it next time.
If it recurrs then consult a gynaecologist for that.
6 months back doesn't have any relation with this current dark color period.