They are both selling for $50. I’m in a dillema because I want a KK, but I also don’t want it full black. I like my Yoyos bright. 
Get the Black/Blue. The KK bearing is useless to play and most in all other ways. I may make suicides fail and the string will only pile up in the middle. I prefer flat over KK.
Thanks for the help! I have heard multiple horror stories about KKs but didn’t know if they were true. Apparently KK’s going wild is the reason for my friends cracked Grind Mutant. I think I will take your advice.