Hi Javi;
Do you know where the first black head came from. Possibilities:
1. He appeared from normal parents as a new mutation. It can happen mainly with dominant mutations. One chance in one million or more.
2. He appeared from mutated or split parents and he is the result of a combination of dominant mutations which are inherited at the same time because their loci are very closed in the same cromossome. It would not be a single new mutation but a combination of 2 known mutations being inherited at the same time. He would be the result of a crossingover (similar to the true lacewing or cin-SL ino crossing over).
3. The black head, and pink head, are also found in other members of the psittacula family (psittacula himalayana, psittacula cyanocephala, psittacula roseata, ...). Is there anything in zorro allowing to think that his ancestors came from hybridizing with one of the species above? (I mean low size, eye iris, psitacines in the wings, different voice or behaviour, ...). It could have been done many generations ago and by back-breeding to normal IRN the black-head could have been "transmited" to IRN.
Which is your feeling?
Something else: even in dominant mutations half of the offspring is not mutated (normal) when pairing to a normal bird. You say that all of them were black head .... so strange

Could you develop further?
Regards
Recio