I hade the same problem as you and it was becouse i had downgraded trough the gateway menu.
This solved the problem for me https://github.com/AuroraWright/Luma3DS/wiki/FAQ-and-Troubleshooting#i-get-a-black-screen-on-boot-with-an-old-3ds-which-has-been-downgraded-from-the-gateway-menu-in-the-past