A young, unsuccessful musician finds a second-hand guitar that unlocks her ability to create amazing sound. But as her powers grow, she discovers that there is a price — and it must be paid in blood.

"I remember everything! I was barely seventeen and I once killed a boy with a fender guitar..." --JIm Steinman. (That quote is from memory)
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An interesting take on the "Sold my soul for Rock N Roll" theme.

I am not sure I would use both young and unsuccessful.

"A down on her luck musician find magical guitar, but soon discovers there is a price- Paid in blood."

(To be honest the original logline is pretty good)