There are several acuses of blacness darkness over nose. Few common conditions causing these are melasma/ seborrheic melanosis/ freckles/ post inflammatory hyperpigmentation. A doctor has to see in person and ascertain exact type and cause of tour pigmentation and also do a wood’s lamp examination to know the depth. Once this is done.. treatment options can be offered depensing on the cause and depth of pigmentation. Start using a broad spectrum sunscreen atleast 20 minutes before stepping out in sunlight and consult a good dermatologist for a proper examination