Dark Grey
That's what I figured and it could well be why the highlights are not as brilliant as they could be. Switch to a white background. Your eye will re-calibrate to it and you will start to boost the highlights slightly. It's an old darkroom printing tip (right out of
The Print by AA): always compare the highlights to "paper white". In your B&W photos, you want a small bit of "paper white" (pure white) to help calibrate the eye of the viewer. Then, you can check for a gradual drop towards light grey - the highlights you want to keep detail in.
Surprisingly, the same thing applies to computer monitors and screen processing. Monitor calibration works to an extent, but can't entirely replace proper "eye calibration".