That's good. It doesn't work on the new 3DS. I haven't ran it, so I can't help ya, sorry. I have looked over the rxTools thread and they do say how to do it, but i haven't gotten around to trying it yet on my 2DS. I suggest skimming thru that thread while waiting for someone with some knowledge to answer.
You need to use the absolute latest rxTools and cdn_firm.exe. In other words, get the 2.2.2 files and use those, remembering to get the new firmware.bin onto your SD from the new cdn_firm.exe.
If you were already using the latest version, make sure you have the correct slot0x25KeyX.bin file also on your SD root.You can probably find this file with a google search, or if not you should be able to figure out how to make it.
If you've followed these and all other steps correctly, there is no reason you should get a black screen. So double, triple check until you get it right.