Why do Muslim women wear blue berk as?

Wearing the berka', or niqab, or forehead veil by Muslim woman is not an obligation per religion. However, wearing it is not prohibited p religion. It is a matter of local traditions and cultures. In other Words, it is found to be popular in some countries while not in others, according to a specific area's tradition. As for the Islamic belief, it is to be favored only if a woman fears to be harmed or to cause harm for others if she is very beautiful, for example.

It doesn't have a specific shape or form, every woman can have her own style, neither a specific fabric is recommended, even a soft transparent cloth would do effect, the aim is to make face expressions not so apparent, but without hindering the woman's ability to look, talk, and breathe.