It doesn't sound like it would be something serious. Some women have darker skin down there. Especially if you have a darker complexion anyway. Unless, you are sexually active (in any way) it couldn't be the results of an std. And if it was an std hiv/aids **maybe** i've never heard of that, but no other stds are known to change the color of your vagina.

Is it your labia or clitoral hood that has darkened.
When did it happen.
Is there any pain when you wash under the clitoral hood?

The blackening of the labia is an indication of the poor blood circulation in
the tissues. It is very natural to have darker inner lips as women age.
Over-stimulation will accelerate the blackening of the inner lips. The color
of the inner lips without masturbation or sexual stimulation is pink. Aging will slowly change the inner lip color. Increasing masturbation and sexual stimulation frequency will deepen the skin color to black around the edge of the inner lips in a short time. I believe the color change is associated with trapping of metabolic waste due to the poor circulation in the inner lips, which rely on the capillary to circulate the blood. This is like the blood congestion in the lips, there is no way to reverse the color change since you can not apply a vacuum massage cup to the tender inner lips, although it can work to improve the blood circulation in the groin and pubis to increase the erectile size (power) of the clitoris and shaft (the penis, too!). The color of the inner lips is a measure (a marker) of the women's sexual experience, but it is very difficult to tell how often a day the women have sex.