Does 1/0 black ink on envelopes mean grayscale or solid black only?

Yes. A 1/0 setup means one ink color prints on the front and the back is left blank, and when that one color is black you get the full range of black, from solid areas to soft grays.

• Grayscale is produced from a single black ink using halftone dot patterns, so photos and shading still reproduce well.
• Solid black fills, black text, and fine black line art all print cleanly at 1/0.
• The 1/0 code reads as front color / back color, so 1/0 = printed front, blank back.
• If you need true multi-tone color or spot colors, step up to full color (4/0 CMYK) instead of 1/0 black.