I had this exact same problem OP!
Went from an all over very shiny healthy auburn-ish brown and asked for balayage just for a change. Provided a picture with a very subtle light-touch balayage, mainly on the mid lengths and ends. The hairdresser was very enthusiastic, talked as though she was experienced in the technique, said she'd done it at "competition level". I'd never been to her before but she filled me with confidence and I couldn't wait to get it done.
It was a flipping disaster.
She basically highlighted my hair with foils, pretty tightly packed, but started them a couple of inches down the hair shaft. While she was doing it I was suspicious as it wasn't the technique I had expected from obsessing over Pinterest and YouTube so I kept bringing her attention back to the photo and questioning what she was doing. Unfortunately she just kept reassuring me, even laughing about how nervous I was but how great it would look, so against my better judgement I just let her get on with it. What an idiot I was 🙄
When she came to check the foils before washing it out, she looked a bit surprised and commented on how easily my hair "lifted". It looked pure white - alarm bells were ringing (for both of us I think)
She then applied two different toners as the first apparently wasn't right. Again, red flags.
The final nail in the coffin of my previously pretty passable hair, was when she dragged a bristle brush through the matted tangly mess. You could literally hear the snapping and see it stretch. It bloody hurt too.
Predictably, the final result was a block of brassy, dirty looking blonde with 2 inches of dark roots.
It was an expensive salon with a good reputation and I paid in full and left with a heavy heart. I didn't have your confidence to ask for a refund, though I absolutely should have!
After a month I went to a different salon to get it sorted and the hairdresser couldn't believe what she was seeing. She rightly stated that it wasn't balayage but highlights, the placement was terrible and the color was all wrong. She then used the proper hand painting technique, bringing a few very fine sections of blonde up to the root to blend in the color, while also bringing some brown down the hair to break up the "block", so I did get the balayage look that I wanted. She also used Olaplex and gave me a protein spray to use at home. She cut a good few inches off too so the condition was instantly improved (it was long so I could afford to lose some). I've been going to her ever since and my hair has never looked better.
Good luck with your next appointment. Research the salon and colorist well. A chain salon with a reputation like TONI & Guy can be a good option as they are obliged to learn the most up to date techniques and keep their training up to date. They also have to specialize in either coloring or cutting - so you'll get someone who definitely knows what they're doing.