A new curse was alleged in this theology because Ham was less discreet than his brothers when Noah, having raised a vineyard post-flood, became intoxicated from over-imbibing wine. Noah in this undignified state apparently felt it was better to dispense with his garments, and the other two sons pulled a robe over him while looking the other way, but Ham (whose name allegedly translates to “burnt black”) looked. Then, the dubious theology went, when Ham got the new curse he became the progenitor of today’s Negros. Southern Baptists repudiated this idea a long time ago, but it figured during the days of American chattel slavery of Negros.
WhatEVER the curse was; it appears that extra-biblical texts are used to justify the reasoning.