But even a "small" amount of input power makes the process much easier because it can be used to bring up the next source of power.

Sure. Portugal is in deed completely “isolated” if it loses Spain, so it needs black start capability if it wants some degree of autonomy.

As far as containing the issue, this was a disaster. On the flip side, this was as good an opportunity to test a black start as any, it went reasonably well, and the network operator was already in the process of contracting two further dams for the ability.