What Causes Black Coloured Skin On The Lips?

hi..i am madhuki from india...my mom is getting a black colour at the boarder of her both upper and lower lips...its being noticed from yesterday...its turning black slowly...can u help me know the reason and also tell me if something is seriously wrong?my mom never smoke and always have two cups of tea daily...coffee is very rare..no drinking..so can u tell me why it is happening..applying lipstick is also very less...