In CMYK mode, when you select black from the colour swatch you get C=0 Y=0 M=0 “K” (key black) 100%: This actually prints as a very dark grey.
To get the most solid, pure black (known as “rich black”), you can put all the CMYK values up to 100%:
There are variations on this: M=100 and K=100 would give a warm red black, C=100 and K=100 would give a cool black, etc.
Note that total percentage of ink above 300% isn’t recommended as it can smear.
