Insert data from blob value into other field.

Morning all.

I'm trying to integrate my forum system with my Minecraft server, I need to create a mysql cron type event to move the value of a custom user profile field into another field within the same table. In this case, I need to move the value of minecraft_username in the custom_fields blob field to the minecraft_username varchar field, I'm planning to run a cron every 5 minutes to update it. How can I go about implementing this? I'm thinking an INSERT FROM type statement, but I'm clueless past that point.