It has nothing to do with menstrual period blood coming out the vagina.
What they are looking for are patches on the outside surfaces of ovary, appendix,fallopian tubes, inestines bowel, bladder, omentum etc which should never bleed but are now showing signs they are bleeding directly, or that there are obvious irritation swellings from where loose blood has landed and caused patches of irritated tissue.
The endo bleeds at the same time as the uterus is shedding its lining. But they are separate transactions.
Menstrual blood which includes old lining from the previous period that didn't shed away properly last time from the uterus, or has been stuck there as a lining for some time is the dark brown sludgy blood. It would not indicate that you did or didn't have endo.
If you had an ovarian cyst that had filled with blood and been there some time, then that turns brown inside and that is a sign of endo that could show up on scans before surgery. and that is not inside your uterus. So please don't worry about the colour of the blood coming out the vagina escape route. They are not interested in that. They are definitely interested in what is happening in the tummy outside the uterus and being "on" right now is very good news if you do have endo and you want them to find it and tackle it during the op.
Just make sure you have enough sanitary towels for afterwards, incase they have decided to take a biopsy from inside the vagina, cervix or uterus. Tampons should not be used for a few weeks to avoid post op infections. Even though generally they are hygenic things, they still advise to use towels rather than tampons while you are healing.
Best of luck for the op. Let us know how things go for you.
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