In Fencing you regularly need to block and riposte with your blade... anything else is illegal!

Sabre is the closest of the Fencing weapons to what we are talking about (slashing) and the trick with parrying there is to have the blocking blade at a diagonal angle from the plane of the attack... that way the momentum of the attacking blade is transferred into a sliding motion along the blocking blade.

Depending on the type of attack, and your desired counter attack or riposte, you can have your blade point slightly towards the attacker - in which case their blade ends up on your guard allowing you greater leverage to push it in a direction you want or to restrict its freedom of movement.
Or... you can have your blade pointing slightly away from your attacker so that their blade slides past you, leaving you inside their guard.

Of course it doesn't always work like that, especially if you've got a good opponent.