In all seriousness Blue Kayn is just as strong as red. Just red Looks stronger because of current meta with tanks. When the enemy team is a team comp consisting of

Just a team full of squishies, Blue Kayn legit has a field day, and literally puts his "farm tool" to work. a blue kayn with
will erase your adc faster than you can say "WHAT THE FUCK RIOT?!", blue kayn is far more efficent in dealing with really squishy comps. Hell I will go blue kayn even if the comp is

For I see 3 squishies I can absolutely destroy if they make even the smallest of mistakes. I really only like Red Kayn in comps where almost everyone is a tank, both forms are good in the right situations. This isn't me dick riding blue form btw, this is just me putting focus on talking about blue form since it's your question.
Q: But Phantom, Full AD Rahst is also good at killing squishy people so why is blue form better at it?
A: Because young padiwan, Rahst does "delayed" damage, his W is a self stun, and his E is slower, and his R instead is mathematically changed to be better against tanks. Blue form, your E is faster/better, your R scales with your AD, and W has no self stun, making him much faster at killing squishies.