Does anyone know if a hair salon will dye the underneath of someone's hair from the natural color to like a blue color?

They'll do what ever you want as long as you pay for it and it's legal.

So if it's a blue you say they will do it because my autn really screwed it up on me

They may try and dissuade you, as it's a little extreme, but I don't see why not.
After all money talks ;)

Its not a birght blue its more of like a royal blue nad belends in with my hair, my autn just missed spots and didn't do the right spot for say so it stands out, and I was looking to see if anyone knew if a hair salon would be able to fix it or something

Keep in mind that many reputable salons will not process you hair if it has been damaged by the previous dye job.