The Darken Compare Data property is used to Darken Compare Data 🙂
Digital waveforms can contain more than just 0s and 1s. In the ATE world, digital waveforms also may contain comparison values like H and L. When a digital waveform with a combination of 0/1s and H/Ls in conjunction with the NI-HSDIO dirvers, the 0/1s would set a line to drive, where the H/Ls would set a line to be in comparison mode (ie read a value and compare it to an expected value). H's in the digital waveform would tell the HSDIO device that it should try to acquire a bit and compare it to a 1.
Given this information, the Darken Compare Data property should be used when a digital waveform is being displayed that has plots which contain both drive and compare data (0/1s and H/Ls). This property was added so that customers could visually see all 4 states on the graph. Before this was added an H and a 1 looked exactly the same (as well as a 0 and an L).
So to see this feature in action, try to draw a single plot that alternates between H and 1.