why am i missing the BC option on the send email notificaiton action? I have SP 2013 on premise and NIntex 2013 forms/workflows

What version of Nintex do you have?  It was added in the 3.0.7.0 release which was done in March 2014. Seem more here  Nintex Workflow 2013 - Release Notes

I have SP 2013 version 3.0.5.0...so that explains it.

In the meantime we created a collection variable, used a dist-list and then used the for each action to blind copy.