The screen on my BV9700 Pro suddenly went blank. The backlight still comes on, just nothing can be seen on it. Still reboots and charges but no startup logo and nothing on screen when it finishes booting.

How do I go about sending it in for repair/replacement?

Seems like Blackview Admin has quit...I can't find any replies for a week or more...

Well, I've contacted Blackview through their after sales contact page and we'll see where it goes from here. But I'm pretty much done with rugged phones. All of the ones I've had had weird annoying little problems that never got fixed.  No O/S upgrades beyond what they were shipped with, and few security updates.  It seems once a new phone is out, the previous model is pretty much forgotten about. Promises that are broken (BV9600 Android 9.0 update? Nope. BV9800 P90 CPU? Nope. The stock firmware will be posted soon? Nope.)

I have a Huawei P30 Pro now. A little more than double the cost of what I paid for my BV9700. It's IP68 rated and it's in a rugged case with a screen protector.  I may not be able to run over it in a truck or use it in the Antarctic or Sahara, but everything on it works and it will be upgraded to Android 10 soon.