It sounds like the dark fur is due to staining from the urine. I forgot to ask what breed she is and if she is groomed regularly. If I examine a light colored dog and see dark stained fur in the vulva area with no signs of itching, licking, redness, discharge, odor, etc. I assume it is due to urine. Fur staining due to this is not a problem. However, it could lead to problems if it is always soaked with urine.
Keeping the fur trimmed around the vulva can help keep the area clean and dry. I have had patients with urine soaked fur that predisposed them to bladder infections.
I recommend taking her to a groomer to get help in this area. It is possible to trim the fur with a scissors, but I do not recommend doing this on your own. I have seen too many pets come to me for stitches because the owner tried to trim them at home. Not because they couldn't cut the fur right, but if your dog jumps or jerks at the wrong moment the skin can suffer the consequences.
The reason I asked about licking, redness, dark pigmented skin is because one or all of these signs can point to a peri-vulvar (around the vulva) infection. To diagnose this, a skin swab can be taken and the sample can be evaluated for yeast or bacteria under a microscope.
Please let me know if there is anything else I can do to help. It sounds like you are a very conscientous owner who pays very close attention to your dog's well-being.