This is fairly recent..... a high-ranking member of the IHA said that if Princess Aiko studied abroad, falls in love with a blue-eyed foreigner and marries him, it will be a DISASTER.
Of course, this statement is general, in the sense that it didn't say whether a blue-eyed foreigner prince is acceptable.
It certainly looks like inter-racial or marriage to a non-Japanese is taboo, or at least not the practice, for the imperial family.