Videoblocks is not a normal footage site. You can`t contribute as a normal contributor and than get a commission on every sale.
They are buying the copyrights to the footage from the artist. They pay you once and then they can do what ever they like with your footage in future. If they want to they could give it away for free and with their prices they almost do already. Unlimited downloads for a price you have to pay for a single video on other sites. Thats a threatning direction towards "everything is free" and devalues our work.
I hope they dont grow to fast because it could destroy sales on the good sites.