I agree with dropping with wherever I also feel works, but a very common question for new players, including me when I first played a year ago was "where should I even go"

Blue diamonds usually just indicate that your chances of finding better loot is higher, or rather, the game considers it an "advanced" loot area

overall it depends on whether or not you wanna place 35th at ayumi, or if youre the kinda player to just go wherever in low tier areas.

I personally drop wherever i feel like works, mostly going for staying alive, and only dropping in ayumi castle if I'm bored and don't want to live, because it never works out, or would've been exponentially easier to go literally anywhere else. it's also important to factor the direction of dropping, expected people, and if you go to the same place too often, mainly so it doesn't feel repetitive for you or your group. simply variety will do just fine, and the blue diamonds don't matter excessively.