You must play through the game normally, saving Black Mask as your last boss fight. Then the final boss
will give you this mission. You'll have to search them out using clues given and your map and each one has a time limit, with 3 junction boxes attached to each one, you'll have to hurry and plant explosive gel one junction box, before detonating all 3 at once. Don't forget to grab the Bat Suit piece, you'll have to retrace your steps to the final box or you'll have to play through again to grab it. However you proceed, I would at least try to find all the Blackest Night suit pieces on your first run, as that suit makes you impervious to damage and will not only speed up your next two runs, but is also th only suit you can get straightaway. It also has it's very own trophy attached to it.