Q: I need to connect my Blackmagic box via thunderbolt..how can I do this with usb-c?

Find a Thunderbolt to USB-C adapter.

Put that phrase, "USB-C to Thunderbolt adapter" into google and you're going to find some interesting reading as well as products.

The only mac right now with USB-C does NOT support Thunderbolt.

This is the 12" retina MacBook.

The short answer is this can't be done.

IT so happpens that the USB-C connector will also be the future Thunderbolt 3 connector, but this won't change the situation for this machine.

This is analogous to what happened with miniDisplayPort and TB: the connector is the same but older mDP machines simply lack the TB  ircuitry.

I don't understand this USB-C vs Thunderbolt thing at all. But, apparently you can go from USB-C at the Mac to various sub-USB-C connections and protocols but you cannot adapt from USB-C at the Mac to A thunderbolt connector at all, now way.

Here's what I found to help confuse you, too:

USB-C and Thunderbolt Display Connectivity

https://thunderbolttechnology.net/blog/thunderbolt-3-usb-c-does-it-all

http://www.macworld.com/article/2930555/thunderbolts-future-on-the-mac-could-be- saved-by-the-usb-c-port.html

http://apple.stackexchange.com/questions/176135/how-to-connect-a-usb-c-based-mac book-to-a-thunderbolt-display

http://www.kanex.com/article/usbc