🚀 go-pugleaf

The poke address that you mention controls the Direct Memory Access
of the graphics processor, ANTIC.  Poking a 0 to this address turns
off the ANTIC processor and thus the screen goes black. There is
another value to poke to this address to turn the screen back on
but since I don't have my reference manual handy, you'll either
have to look in a manual (if you have one) or examine the program
listing for another poke to 559 (which may have been accidently
left out of the listing and is why your program won't work).
The purpose of turning off the screen is that when you redefine
the character set, the screen becomes rather messy during this
redefinition process, and a black screen is better than a messy
screen. You can leave the poke 559,0 out if you like.  It is
there only for cosmetic purposes and is not neccesary to the
execution of the program. I hope this has helped you. I have
owned an ATARI since 1980 and am pretty familiar with this
machine.