A feature that I personally always wanted.
It's honestly pretty distracting otherwise...
Also asked for some times:
https://www.reddit.com/r/blender/comments/crb3i9/is_there_a_way_to_remove_the_black_border_on/
https://blender.community/c/rightclickselect/Psfbbc/
https://blender.stackexchange.com/questions/153527/black-borders-on-reference-images
Diff against master dedd9ce0aa17fb733c8b1c51b072555fcc4f2fd1