I can think of three times when the blodgers come - at the beginning of the level before you repair and move the barricades, and then in the room where the blodger comes out of the box on the right side along the railing. At first it goes nuts and then it settles down and aims right at you.
Put it to use by lurking under the flat brown slab or hide behind the pillar, and when you see a red circle surrounding yourself, that means it is coming so jump out of the way. It will hit the pillar that supports the slab causing it to fall and will create a ramp to the next level. You will need to do this a total of two time to ascend to the top of the Slytherin stand. The exit is at the very top.