You were told wrongly. Some dark Gray advanced DIY paints that contain a preponderance of Metallic particles and that acheive the shade of dark Gray via RGBY suspended elements do provide splendid images...but ain't a one of 'em actually "Black".
Yes, it's true that you can produce a very watchable image on a Black Surface, but the amount of lumens required is disproportionate to the size of image you can obtain.
If you shot a 82" diagonal image (16:9 @ 40" x 71") you'd get 68 foot lamberts....on a 1.0 gain surface.
But on a Black surface...something akin to a 0.1 gain surface....... about 2 foot lamberts....if that.
The smallest image the 750 can manage is 57" diagonal (28" x 50" @ 3' Throw) and at 0.7 gain you'd get 97 fls
With 0.1 gain...maybe about 4 fls at the uttermost. That would be fortunate.
Still, you can try what you want...but if you were wise, you'd haul up short and instead settle for a ultra dark Neutral Gray.....a image size as small as you can accept, and a Throw distance that optimizes your lumen output.