Hello,
I think my server got blacklisted because we added some plugins to it.
I wanted to ask how can I be sure that my server is blacklisted and how can I get "unblacklisted"
Thank you.

Change your IP or contact FP support.

why do they ban servers? 

Chris94803

why do they ban servers? 

Generally for abuse. Faking stats, redirect servers, modded (beyond admin tools) under Community, etc.