Talking of RNG, I’ve only ever received one 5* costume (Joon). playing 3½ years, I’m level 80, have done all the Masquerade quests since it came out and have purchased a fair number of the offers for Masquerade, and on average collect 10 keys between each event.
Then one day I got a 5* costume from HA and it was another Joon.
Anyway, I hope you get your blue soon! That is one colour I always had too many of.