No need to get the new 100 bill, but do try your best to get 100's that are in very good condition... they don't have to be mint, but no torn or heavily worn bills because they will be penalized with a lower exchange rate. Also, smaller bills will also be penalized with lower rates.

The "blue" market rate is part of the grey market economy, meaning that while the practice of trading US$ is tolerated, it is not legal and therefore you have no formal legal recourse if you decide to operate in this market... hence the above advice to understand the details of what you are getting into and do proper risk management.