[Q] Z1 c903 Bootloader locking. Help!

Hello all! Im new in xda forum, so i will be greatefull if you help me with my Sony Z1 phone. I have Z1 c903 on android 5.0.2., and I was mad on sony becouse they didnt make update to 5.1, so i was looking on forum how to root and install some custom rom and make my phone updated. So i find on some thread that i should unlock bootloader, and i followed instructions and unlocked it, but without making some backup or anything else. So i have seen that there is official update for sony and i want to get back to lock bootloader. But when i enter flashtool and enter "blu" i click on relock bootloader, and start phone type *#*#service#*#* it say that my bootloader is still unlocked. How should i get back to locked bootloader or make official update to 5.1. Please make it more clear becouse i am pretty noob for android development and rest. Also i want to add that i have not rooted my phone or install some custom rom or other, i just have unlock bootloader and thats all. Could i get back my "DRM keys" and could i get back my ofiicial android. Thanks in advance.