More viable veins on breasts/nipples including little dark blue ones, what could be causing this? (excluding pregnancy and menstrual cycle)

Probably normal : With age under 20, it is unusual to find very serious breast abnormalities. Remember that breast tissue may swell at various times throughout the menstrual cycle. Some superficial veins may become more prominent throughout your cycle. It is not normal to find a mass or bump that you can feel or if you have nipple discharge. If you have pain and specific part of your breast, it's good to see a Doc.