To clarify, to get this achievement, you have to accumulatively score 10 points in blitz as an attacker.
You do NOT have to win 10 full rounds.
To score points as an attacker in blitz, you must wait until you are on the team that has the attacker role, and get into a certain area of the map that the defenders will be trying to keep you away from, as soon as you get to the marked circle, you score a point. Rinse and repeat 10 times for the achievement.