When displaying past (historical trend) data on the SMARTDAC+ recorder, what is the meaning of the 4 numbers displayed in blue and black in each channel of the numerical value display area?

The following numbers are displayed in blue and black in the numeric display area when historical trends are displayed.

Blue: the maximum/minimum values of the data of the overall waveform currently being displayed (on screen).
Black: Maximum/minimum values of the data on the left edge of the scale.

For details, see section 2.2, "Configuration of Measured Data Display" in the user's manual.