Not sure if you are still having issues, but from what you said with the blue light fading out it sounds like an exploit was used on it called arm9loaderhax, in which case the system will refuse to boot without that file on the root of the sd card.
https://github.com/AuroraWright/Luma3DS/releases/download/v7.0.5/Luma3DSv7.0.5.7z
If you download this and put the included arm9loaderhax.bin file on the root of the sd card, and then after inserting the sd card back into the 3ds try to boot it it should work