Checked up dto's suggestion and this seems to fit. For those interested the story is:
Black Zero appeared in Superman #205 (1968), in a story entitled "The Man Who Destroyed Krypton"
(
http://en.wikipedia.org/wiki/Black_Zero#Pre-Crisis_Black_Zero).
Following the links, there was a suggestion that Black Zero cancelled out the attempt by the Green Lantern Tomar Re's to prevent the destruction (as seen in Superman #257 (1972)), meaning the two sets of interference cancelled each other out, and Jor-El was right.
However, looking at
http://www.supermansupersite.com/1972.html this doesn't seem to fit to well?
Thanks, Jimmy-Neutron