Most Xposed changes, via the modules, require a reboot to make them apply. I also keep my lock screen widget (Dashclock) maximized so I never even see the carrier label in the first place. Just changed it and then rebooted to be sure it was working, change applied just fine after reboot. Now says GO BRONCOS!! in Holo Blue. Only other suggestion I have, if you want "blank" text in carrier label:
Make your lock screen wallpaper completely black
Set text color in XBlast to be black for lock screen label; you may need to type a symbol or a word as well in order to override the "blank for default" option
Reboot
Carrier label should now be the same color as background and therefore "invisible"