yes, Blackbox is free to join and use.
Basically you join, create an account and upload videos, by FTP, of between 5 seconds and 60 seconds in length.
Part of the upload process is to add details during any upload.
Your video clip is viewed by Blackbox who decide if it is suitable quality and if it is they submit the Video Clip to Stock footage agencies, who advertise the clip on their websites.
Hopefully advertising agencies find the clips and buy the rights to Download the video Clip.
Blackbox gets paid for the clip, deducts their cut and sends you the balance.
*** This is a very simplified version of the process and I urge you to read everything regarding Blackbox guidelines - there are user guides and rules to follow ***
I will help as you begin if you need me to.
Good Luck and welcome aboard should you join Blackbox!