What's the blue pentagon for in Pokemon XY?
It's apparently a Kalos Region checker. Pokemon native to Kalos have it. It is not, as some say, a hack checker. This is easy to confirm since Pokemon Transporter straight-up won't import many hacked and some not hacked pokemon.
With the introduction of Pokemon Omega Ruby and Alpha Sapphire, the blue pentagon does not mean that the pokemon is native to Kalos, but it is a pokemon obtained in either XY or ORAS (6th generation). Thus, any pokemon except certain event pokemon can legally have a blue pentagon.