It's also possible for parents who are both blond haired to have dark-haired children (that happens if the parents both have heterozygous genes).
But. my opinion is still agreeing with you. Ileana and Mircea don't look much like Carol, Elisabetta, Mignon and Nicolae.
I wonder where Hannah Pakula got the information that Ileana is genetically Ferdinand's daughter. From a DNA test?