They worked at a power plant, a place where dumb mistakes can cause explosions and kill people. The power plant wasn't racist and hired blacks into the labor department, but because it was just manual labor that department paid worse than the other more technical departments.

When SCOTUS found against the power company they sent a clear message that merely being a technical, safety-critical job was an insufficient basis to establish a need to test people for intelligence. And as it's hard to argue that testing isn't needed for people who could cause massive power outages but is for <job X>, that was widely interpreted to ban such aptitude testing for any kind of job.