It seems like I cannot adjust the dashcam to be fully behind the rearview mirror (such that the dashcam is not in the view of the driver at all) with the Blendmount.
This is possible with the adhesive tape.
Is there some trick to better position it with Blendmount?

 

I had to fiddle with it but was able to locate behind the mirror.

 

What is your part number?
I got this: https://blendmount.com/aluminum-dash...750s-bbv-2032/
But the unit I received did not have the middle extension so it could not extend all the way back:

 

My part number is the same as your ordee. I suggest contacting blendmount.

Item # Description
BBV-2032 Aluminum Dashcam Rearview Mirror Mount, BlackVue DR900X/DR900S/DR750X/DR750S, BBV-2032