The difficulty is still the same: for each piece of the puzzle (substations, high voltage power lines, transformers, generators, loads), they have to wait until enough power is available at the connection point, and carefully manage the power balance so it's neither too much nor too little (and if they get it wrong, things trip offline and they have to start all over), while the grid is in a degraded state (often meaning no alternative paths, so a single fault can set them back several steps). The only difference being that the black start "generators" are these still working international links (which could be very far from the important parts of the national grid).

It might be faster to instead black start several independent power islands in parallel, and connect them together as a final step. At least in my country (Brazil), that's how it's done for large-scale blackouts, even when some of the country still has power; it was done that way for the partial blackout in 2023, and there's a written procedure on how to do it (which is available on the operator website, if you know where to look). In 2023, some areas failed to black start for one reason or another, and had to wait for power from the outside; other areas managed to black start as expected, and were then synchronized with other areas until everything came back together.