Descartes Merge vector with white background

We are working in PowerCivil with white background color. When testing Descartes Merge command to create georeferenced rasters, all black (color 0) elements were excluded from the raster. 

Is there a setting somewhere that allows me to create a georeferenced raster with white as background color?

I have found a configuration variable named "_DC_TRANSPARENCY", but I have not found any documentation on that variable.

In the example below, only non-black colors are saved to raster.

ORIGINAL

OUTPUT FROM MERGE TOOL

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