This can be achieved at any time. Get close to an enemy, throw a flashbang with
to blind yourself, aim down the sights with auto aim enabled and you should hit and hopefully kill them. Killing an enemy while being blinded by a teammate or enemy flashbang will also count, and will likely be earned through normal play.
Feel free to ask any questions, and add me on TT/ TA/ PSN/ XBL as Danokitty. Happy hunting!