Probably nothing. It is not uncommon for inner labia to be darker than outer labia. The skin is thinner for inner labia and one is likely to see more underlying vascular or blood rich tissue. See your doctor if this new or you are not sure.
Normal variant. The labia minora are the inner folds nearest the vagina. It is quite common for these folds to have a blue tint. As you age the tint can become more prominent. It is very concerning if there are areas that become very dark or are thickened or nodular you, then you should see your gynecologist for evaluation.