It is actually quite easy: "Black" print does not mean that 100% of all pixels are actually set. Print pixels are never perfect squares, so even if the printer only prints half of the dots, the print is still dark enough. If not, it could print 70% or 80%, but lets stick to 50% for ease of argument.

So instead of

XXXXXXXX
XXXXXXXX
XXXXXXXX

it would print

X X X X
 X X X X
X X X X

For you, it would still be a "roughly black" spot (keep in mind these 8x3 pixel are 0.032mm wide and 0.012mm high on good laser printer).

Would you notice if the pattern was slightly different, like

X X X X
 X XX  X
X X X X

Make a bonanza of those small changes nobody can see, and you can hide thosands of bytes of data in those patterns on any printed page.