If, like me, you use BootCTR9 for a boot animation with B9S, then what you'll want to do instead, is download the Luma 9.0.7z here:
https://github.com/AuroraWright/Luma3DS/releases open the zip, extract boot.firm, rename it to luma.firm and put it on the root of your 3DS' microsd card, overwriting the old one, since the animation is instead being loaded from boot.firm, and the luma boot is being redirected to luma.firm it's possible you may have your config set to load the luma .firm as a different file name, but I think luma.firm was the default to choose, so that's what I stuck with, but if you have a boot animation with B9S, and not a luma.firm file already on your root, then locate whatever the .firm is that you have it being redirected to for luma, and rename the new boot.firm to that name instead.
Edit: actually after looking through the default config file that bootCTR9 has, it would appear that luma.firm isn't the default that it redirects to. If you use bootCTR9 as a boot animation, look at your boot_config file, and you should see something like this for where luma is being redirected to:
[DEFAULT]
path = /luma.firm
delay = 1000
splash = 4
splash_image = /anim/0/anim
screenBrightness = 0x30
(Your setup may vary), point is, look for where the path is, and that's what you'll want to rename the boot.firm file to and where you want to place it, in my case, I had to rename to luma.firm and put it on the root of the card.