I'd go back. Poor Dsis had hers done at the weekend - she asked to go a slightly darker blonde and they dyed it grey . Twas hideous, so bad infact that I made her go back immediately eventhoughitmeantwrestlingwithmy3nieces/nephewforthemorning. That was how bad it was. Still not right even now.
Its BLIMMIN hard to "match" mid-mouse, as you describe it. She's used a semi, it will almost undoubtedly fade a bit to a more natural hue. I'd ring her in the morning and state your concerns - see what she says. Ask what she used if you are game. (Loreal Diacolour Richesse? Wella? Etc). Then we have google-ability). But I'd suggest it WILL fade.
If you ring tomorrow, register your "concern", and its still bad in a week or so, you could most definitely go back.