Is there any concrete reason to call BLAKE more secure than BLAKE2?
I guess that reducing the rounds is indeed the main reason to call it less secure. Generally attacks are over a specific number of rounds. If better attacks are found then the reduction of rounds could mean that Blake2 gets broken while Blake is not.

How secure something is is up for discussion for the simple reason that we cannot predict the future. I guess that the authors were confident enough in the overall structure of Blake to make the changes.

Neither algorithm has been broken so up to now they are right and Blake2 is as secure as Blake. The full rounds version of neither is affected by the attacks. 

However as an attack is more likely to affect Blake2 you can still argue that Blake2 is less secure if an uncertain future is taken into account. With so few rounds broken you can argue that the difference is minimal (AES has many more broken rounds) but it's there.