Don’t think it’s possible my guess is there is already a blue flames devil fruit

Sabo is in deep waters right now and it is possible it will spawn up elsewhere but it won't change its colour in fruit form and i don't think blue flames are shown since the mera mera no mi is already burning at it highest temperature i guess

But ace has shown the ability to turn his flame green

I think blue flames to represent Sabo's power increase is quite fair...

Blue flames is basically Phoenix fruit

No phoenix fruit is a very special kind of healing fire, not supposed to be hot I think.....

Blue flame from Sabo I'd imagine be more indigo in color than Marco's flame (blue-greenish)

Idk lol