If the text is dark blue, running the Red and Green guns from the same signal at full blast should yield black text. If the overall blue tint is very light, tinting it in all three Chroma ought to yield an even white or maybe a light gray desktop. White results from a limited excitation of the phosphors on a CRT in either Monochrome (grayscale and B&W) or Color which sets off the desktop from the surrounding blank areas of the CRT. LCDs don't have the display area limitations of a CRT, though some black margins show at the edges.

edit: I wonder if I might have that backwards, maybe white is all three guns running full blast? Dunno if CRT phosphors are an additive or subtractive color model offhand.