Can I stop software dimming from making the screen completely black?

Yes. Set Redefine 0% software brightness level to a value above zero. BetterDisplay then maps the software-brightness slider into the range between that floor and 100%, including the software component of combined brightness.

Open Settings > Displays > [display] > Device Control > Image Adjustments > Image Adjustment Settings… and find Redefine 0% software brightness level. Software-based video adjustments must be enabled for this option to appear. For example, a value of 10% makes the bottom of the software slider use a 10% software level instead of complete black. It is a software adjustment, not a measurement of the panel’s light output.

This differs from Combined brightness - minimum allowed hardware brightness level, which sets the lowest hardware level used by combined brightness before software dimming takes over. Changing that hardware floor alone does not prevent the software image from reaching black.

The software floor also does not override separate screen-off actions. If enabled for a compatible monitor, Turn off screen when combined brightness is set to zero can still request that its screen switch off. Review that option under Device Control Settings… if your aim is to keep the picture visible at the bottom of the combined slider.

See dimming below the hardware minimum and restoring full dimming after wake for the related settings.