Who does OP think he/she is... The master of warnings? Why the "Last warning"? Can't we the normal poster not issue warnings. Does only OP's warning count?
Issue: I downgraded, but my 3DS screen is completely black upon reboot!
Solution: Try removing the SD card, or you can reboot into recovery mode. To do this, hold A+R+L+Up as your 3DS turns on.