Significant parts of the eastern interconnect successfully islanded the blackout areas during the cascade failure in 2003, while the blackout was huge the interconnect itself was still up and running for (geographically) most of it, so thousands of generators worth of power were available to feed in via dozens of major transmission paths to re-energize the faulted areas.

In a real "black start" none of that external capability and coordination exists. Imagine the entire US and Canada east of roughly Kansas being dark.