Oh I sure hope he stays as solid white as possible, yes that it a tiny touch of black on the belly, I hope that goes away.
I love your avatar of your betta Hatsunemiku what a beauty.
I sure hope he turns out to be a HM, I have kept many many bettas in my time but never a HM so this is myfirst one.
I just love white bettas no matter what kind of tail type, they are my favorite.
Is there any foods that one should avoid feeding white bettas?
I will watch for the temp too, I have just gone through a nightmare with that. I keep my boys in 1 gal tanks, 4 of them are room temp and the two main ones are heated. During the summer months the tanks heated up quite hot even with central air we have. My beautiful Doubletail blue/yellow his fins wer shredded looking and still are trying to recover. Now I don't know if that was from the higher water temp or him acting like a moron at the other betta beside him and getting his fins are taggered. Since I have put him in a unheated tank his fins are growing back very nicely now so this will be a wait and see.
The temp for them in unheated is 69-70 and in heated right now as its cooler here winter on the way it is 78 so I guess trial and error for this.