Thanks for writing.
Ruling out pregnancy
by a home urine test should be your first step.
In case the urine test is positive, do get an ultrasound
to rule out ectopic pregnancy(pregnancy outside the uterus
) which can present with delayed periods followed by vaginal bleeding in the form of altered blood discharge.
If your test is negative then the black brown fluid can be explained by starting of menses. In delayed periods, blood take time to exit and such a blood gets altered to dark red or brown.
I hope I have answered your query.