Guys, I had the same issue but I worked out a solution. I was smashing my head, wondering what the hell I did wrong. I followed multiple tutorials to a T. The problem is that these tutorials are missing information.
If you are getting a black screen from autoboot, and or unable to launch rxTools from the HBL, this post is for you.
1. Download the starter kit and put it on the SD card.
2. Download rxTools 3.0 or above, I am using a nightly. Make sure you put firmware.bin in the /sys/ folder of rxTools if you are using a version past 3.0.
3. Make sure you have your dbs folder (next to the exdata folder) properly configured. If you just downgraded, and or formatted your System NAND, it is likely it is not properly setup. Create a folder under Nintendo 3DS/nonliterallyrandomcrap/nonliterallyrandomcrap called dbs. It should be in the same directy as a folder called "extdata", create two files inside the dbs directory: import.db and title.db. You can do this by making a txt document and renaming it.
Go into System Settings --> Data Managment --> 3DS --> Software. Click "Reset" when prompted.
4. Change your theme to something, or at least visit the menu where you change your theme. This creates important data for homemenuhax
5. Launch ninjhax 1.0. This is very, very very important. Do not load ninjhax 2.0 or ninjhax 2.5.
6. Open rxTools from within ninjhax 1.0! Let it create necessary files, and you are presto.
7. Load the latest ninjhax and install menuhax to your liking.
8. Now replace your homebrew launcher boot.3dsx with your rxTools 3dsx. Personally, I used the one packaged in
RxToolsHomeMenuHax.7z found here:
https://gbatemp.net/threads/guide-rxtools-autoboot-on-o3ds-using-homemenuhax.398911/
As I was not sure if the one provided worked with menuhax just renaming it to boot.3dsx
Finally, you will no longer get a damn black screen.