Regarding Blackmagic on linux you have to use gstreamer through pdgst.

There are two plugins "decklinsrc" and "decklinksink", this is working great !

I think they are in the "bad" plugin set

If this works the same in windows, perhaps you have to fisrt set the mode of the blackmagic before turning gem on. By setting up I mean selecting the appropriate input and video resolution.

And I confirm, never change settings while using it, unplugging the blackmagic input whit gem on freezes my computer

Etienne