Blood Lust

If target creature has toughness 5 or greater, it gets +4/-4 until end of turn. Otherwise, it gets +4/-X until end of turn, where X is its toughness minus 1.

Masters Edition III

Fifth Edition

Target creature gets +4/-4 until end of turn. If this reduces that creature's toughness to less than 1, the creature's toughness is 1.

Renaissance

Fourth Edition Foreign Black Border

Fourth Edition

Target creature gets +4/-4 until end of turn. If this reduces creature's toughness to less than 1, creature's toughness becomes 1.

Has changed slightly in functionality, with the old continuous nature of the toughness reduction gone away; the toughness change remains static. For example, if you have three Swamps and Nightmare on the battlefield and then cast Blood Lust on it, it will become 7/1. If one of your Swamps is subsequently destroyed, your Nightmare will become 6/0 and be put in the graveyard.