Black is very confusing colour because it is a sign of “Apshagun” on one hand and on the other hand it is “NazarkaTeeka” which keeps bad spirits away. Some people use to wear black thread because they find it holy thread and say it is the favourite colour of God “Shani”. Whereas black cat is considered as a sign of mishappening.
Granny’s “TOTKE” were awesome, every grandmother have their own superstitious belief regarding black colour. Now, the black thread has become a fashion statement, especially for girls, although boys also go for it. Usually, girls wear black thread on their ankles. They might feel it out-dated to wear it around their neck so they do prefer wearing it like an anklet. It is a combination of superstition with fashion. It gives you quite a trendy look along with protection from evil eye.
Black is no doubt a very attractive colour. Black is the combination of heat and hot. It suits on almost everyone (apart from the hot sunny day). There are various reasons to wear black thread on the ankle like, Some people also wear black thread just to gain attention (especially girls) while others prefer it for prevention of diseases.
Girls wearing tight skinny jeans, high heels, and black thread just little up towards their sandal laces was a just a style symbol like you have noticed tattoos.
Basically, this black thread has no significance at all. These all are the myth until you call it fashion.