What is the main function of a reverse polarity diode, what would happen if it was blown

Hi,
a diode in reverse polarity could be used e.g. for protecting some circuit against improper polarity of supply. It could be connected in serial - but it is in forward polarity - (disadvantage: voltage drop) or in parallel - reverse polarity - with a fuse in serial to supply. This way is often used with Zener diode - protection against overvoltage in addition. Try to google e.g. "how to protect against improper polarity". For this type of protection could be used special types od diodes - transils - they could be one- or bidirectional. In case of a diode failure, the supplied circuit may not work - it depends on the way of diode connection and type of its failure - short circuit or disconnection. But there is many diode application: try https://en.wikipedia.org/wiki/Diode https://en.wikipedia.org/wiki/Diode#Applications
Jiri