Elden Ring has lots of Questlines that you need to complete in order to progress further into the game. Most of the questlines required the help of the NPC or interaction with them.
Similar to once you defeat the Radahn and then interact with Blaidd it says he will meet you in Nokron. But when you visit Nokron the Eternal City, Nokron is nowhere to be found. This simply Nokron is never able to reach the Eternal City.
Where You Can Find The Blaidd in the Elden Ring
When you interact with Blaidd just after defeating the Radahn you will find Blaidd at the Forlorn Hound Evergaol in Limgrave. Because the NPC called War Counselor Iji has imprisoned Blaidd and they he cannot able reach the Nokron.
Once you arrive in Forlorn Hound Evergaol you will hear the Blaidd howling sound you need to interact with the Evergaol and you will be given the option to free Blaidd or not. So its totally depends on your choice.
However, you want to continue the Ranni questline then you need free Blaidd. If you choose to free him, he will express his gratitude towards your efforts. After you can find Blaidd in the Ranni Raise tower for completing the Ranni questline.
So after defeating the Radahn you need to approach Forlorn Hound Evergaol in Limgrave and not Nokron the Eternal City and free Blaidd to complete the Ranni questline.
This is the end of the Blaidd After Radahn location guide for more helpful content does read our Elden Ring Guide.