If you look under the rules for the event it only has the four colors listed under planeswalkers allowed. Karn can only use the colors allowed in events, thus blue not allowed for Karn, but Kiora has full use of her colors.
I would like to know why we can’t use mono-blue Planeswalkers for this event. It might be a bug after all. If not, it should probably be mentioned in the Sneak Peek because everyone’s used to having access to all colors in every PvE event.
I noticed this too when I tried to select Tezz I for the 3rd fight. Then I read the event rules after someone in my team mentioned it. I probably should’ve thought to read them first but hey, I have to learn everything the hard way.
In either case, all 4 fights, for me anyway, used the same “Shard of the Twelve Worlds” special support.
Anybody else?