How does contrast filtration work to make blacks blacker?

Ole said:

...the Ilford EM-10 is a cheap and easy substitute. I
have made a "calibration table" (really a graph) for mine ..

Click to expand...

The EM-10 is a very small spot enlarging meter. I've
calibrated mine against a 21 step step wedge. Now
it can be used for close, not exact, densitometry.

To use it as a densitometer be sure all conditions
for reading are the exact same as when calibrated.
The device is non-linear. I'll likely recalibrate just to
be sure the POT, gain control, is still on track. Dan